configure: fix OpenGL autodetection on OS X

Was broken from 746b5e6 since the OpenGL headers are under OpenGL/ and not GL/
on OS X. Thanks to @Kovensky for the initial patch.
mp_msg: don't change text color for normal output

Normal text was set to gray foreground color. This didn't work for
terminals with white background.

Instead of setting a color for normal text, reset the color attributes.
This way, only errors and warnings are formatted differently.

Also change the default color for MSGL_HINT from bold white to yellow.
input: make input command deprecation warnings visible

Some time ago, all old special-cased commands (like "volume 1" to change
volume by one) have been removed. These commands are still emulated
using simple text replacement. This emulation is done to not break
everyone's input.conf, especially because the input.conf provided by
standard mplayer* still uses the old commands.

Every use of a deprecated command prints a replacement warning, which
was visible only with -v. Make these warnings visible by default.

There's actually not much reason to do this, but since commands like
"volume 5 1" don't work anymore, it's better to be verbose about this.

Also simplify the replacement for "vo_fullscreen".
command: export VO video width/height as properties

Add new properties "dwidth" and "dheight", which contain the video
size as known by the VO (not necessarily what the VO makes out of them,
i.e. without window scaling and panscan).
mplayer: cosmetics: make some code more readable
mplayer: make --frames=1 work for audio

This has the same (useless) definition as frame stepping in audio-only
mode: one frame means one playloop iteration. (It's relatively useless,
because one playloop iteration has a random duration. But it makes
--frames=1 work, which is useful again.)
core: output --playing-msg message only after at least one frame is shown

This way it's possible to retrieve correct information about video, like
actual width/height, which in general are available only after at least
one frame has been sent to the video output, such as dwidth/dheight.

mpv_identify.sh becomes a bit slower, because we let it decode enough
audio and video to fill the audio buffers and to send one frame to the
video output. Also, --playing-msg isn't shown anymore with --frames=0
(could be fixed by special-casing it, should this break any use cases).

Note that in some corner cases, like when the demuxer for some reason
returns lots of audio packets but no video packets at the start, but
video actually starts later, the --playing-msg will still be output
before video starts.
mpv_identify: allow specifying mpv binary via MPV

Useful for testing.
mpv_identify: add --quiet to mpv command line

Prevents the status line from being printed. Otherwise, the status line
is always printed due to --frames=1, and it's visible on the terminal
because it's printed to stderr.
m_option: pretty-print floats with 3 pre-decimal digits instead of 2

There were complaints that ${fps} was printed as e.g. "23.98" instead of
"23.976". Since there's no way to format floats exactly _and_ in a user-
friendly way, just change the default precision for printing floats.
demux_mkv: set correct aspect ratio even if DisplayHeight is unset

Fixes the file test2.mkv from the official Matroska test file
collection.

libavformat does the same thing.
demux_mkv: fix skipping broken header elements

Fixes test4.mkv from the Matroska test file collection.

demux_mkv_open() contains a loop that reads header elements. It starts
by reading the EBML element ID with ebml_read_id(). If there is broken
data in the header, ebml_read_id() might return EBML_ID_INVALID.
However, that is not handled specially, and the code for handling
unknown tags is invoked. This reads the EBML element length in order to
skip data, which, if the EBML ID is broken, is entirely random. This
caused a seek beyond the end of the file, making the demuxer fail.

So don't skip any data if the EBML ID was invalid, and simply try to
read the next element. ebml_read_id() reads at least one byte, so the
parsing loop won't get stuck.

All in all this is rather questionable, but since this affects error
situations only, makes behavior a bit more robust (no random seeks), and
actually fixes at least one sample, it's ok.

libavformat's demuxer handled this.
demux_mkv: improve robustness against broken files

Fixes test7.mkv from the Matroska test file collection, as well as some
real broken files I've found in the wild. (Unfortunately, true recovery
requires resetting the decoders and playback state with a manual seek,
but it's still better than just exiting.)

If there are broken EBML elements, try harder to skip them correctly.
Do this by searching for the next cluster element. The cluster element
intentionally has a long ID, so it's a suitable element for
resynchronizing (mkvmerge does something similar).

We know that data is corrupt if the ID or length fields of an element
are malformed. Additionally, if skipping an unknown element goes past
the end of the file, we assume it's corrupt and undo the seek. Do this
because it often happens that corrupt data is interpreted as correct
EBML elements. Since these elements will have a ridiculous values in
their length fields due to the large value range that is possible
(0-2^56-2), they will go past the end of the file. So instead of
skipping them (which would result in playback termination), try to
find the next cluster instead. (We still skip unknown elements that
are within the file, as this is needed for correct operation. Also, we
first execute the seek, because we don't really know where the file
ends. Doing it this way is better for unseekable streams too, because
it will still work in the non-error case.)

This is done as special case in the packet reading function only. On
the other hand, that's the only part of the file that's read after
initialization is done.
core: always pass data via packet fields to video decoders

Makes the code a bit simpler to follow, at least in the "modern"
decoding path (update_video_nocorrect_pts() is used with old demuxers,
which don't return proper packets and need further parsing, so this code
looks less simple now).
gl_common: split into platform specific files

Do this instead of stuffing all x11/cocoa/win32/wayland specific code
into gl_common.c. The cocoa specific parts could probably go directly
into cocoa_common.m, possibly same with wayland.

Also redo how the list of backends is managed. Get rid of the GLTYPE_
constants. Instead of having a big switch() on GLTYPE_, each backend
entry has a function pointer to setup the MPGLContext callback (e.g.
mpgl_set_backend_x11()).
vo_opengl: split into multiple files, convert to new option API

gl_video.c contains all rendering code, gl_lcms.c the .icc loader and
creation of 3D LUT (and all LittleCMS specific code). vo_opengl.c is
reduced to interfacing between the various parts.
sub: don't crash on GBRP video

mp_get_yuv2rgb_coeffs() will crash if the colorspace is explicitly set
to RGB.
gl_video: move video image fields into a struct

This is a bit cleaner. Also don't repeat the chroma shift calculations
over and over, but store the image size instead, which is simpler and
will give us a chance to fix display of non-mod-2 image sizes.
gl_video: make it possible for planes to have different formats

Preparation for NV12 support.

Also adds support for IMGFMT_YA8.
gl_video: add support for NV12

There's really no reason for this, but it feels nice being able to
support a weird pixel format.