vo_null: don't reject hwaccel formats

This is not strictly needed anymore. (On the other hand, it's not really
possible to do hw decoding with vo_null, because the VO is still
responsible for opening the hw decoder API, but that's another story.)
input: improve some comments

I have the feeling some of these were written before everything was
changed all over again.
input: don't emit command when using multiple mouse buttons at once

This is for key bindings that use multiple mouse buttons at once. (Yes,
this is weird, but MPlayer always had this feature, and apparently
there are people using it!)

Before this commit, clicking another mouse button while still holding
the previous mouse button forced the command bound to the previous
mouse button to be emitted. This is usually needed to make sure the
input consumer (the player and the OSC) stays in sync with the actual
mouse button state. If there's no command sent, the OSC in particular
would think the button is still held down. However, sending the command
is undesired behavior if you want to use these multiple-key binds.

Solve this by emitting commands in this situation only if a key down
command was sent earlier. Since mouse button key bindings are normally
executed on key-up only, this happens with special commands like
script_dispatch only (used by the OSD to track mouse buttons, but
also used for other OSC bindings).

See github issue #390.
mp_image: deal with FFmpeg PSEUDOPAL braindeath

We got a crash in libavutil when encoding with Y8 (GRAY8). The reason
was that libavutil was copying an Y8 image allocated by us, and expected
a palette. This is because GRAY8 is a PSEUDOPAL format. It's not clear
what PSEUDOPAL means, and it makes literally no sense at all. However,
it does expect a palette allocated for some formats that are not
paletted, and libavutil crashed when trying to access the non-existent
palette.
vo_opengl: add support for rectangle textures

This allows vo_opengl to use GL_TEXTURE_RECTANGLE textures, either by
enabling it with the 'rectangle-textures' sub-option, or by having a
hwdec backend force it. By default it's off.

The _only_ reason we're adding this is because VDA can export rectangle
textures only.
build: reject broken roaraudio sndio emulation

roaraudio has some sort of sndio emulation, but apparently its header
file is either blatantly broken, or an old version from the past. The
sio_onvol() function has the wrong return type (void instead of int),
and the SIO_DEVANY symbol is missing entirely. This broke the build,
because the configure check was successful anyway.
gl_video: support packed YUV formats with Apple extensions

This adds support for packed YUV formats (YUVY and UYVY) using the extension
GL_APPLE_rgb_422. While supporting this formats on their own is not that
important (considering most video is planar YUV) they are used for
interoperability with IOSurfaces.

Next commit will use this formats to render VDA hardware decoded frames through
IOSurface and OpenGL interoperability.
vo_opengl: support for vda hardware decoding

The harder work was done in the previous commits. After that this feature comes
out almost for free.

The only problem is I can't get the textures created with CGLTexImageIOSurface2D
to download properly, thus the code performs download using some CoreVideo APIs.

If someone knows why download of textures created with CGLTexImageIOSurface2D
doesn't work please contact me :)
options: add option to disable using right Alt key as Alt Gr

mpv was hardcoded to always consider the right Alt key as Alt Gr, but there
are parituclar combinations of platforms and keyboard layouts where it's more
convenient to treat the right Alt as a keyboard modifier just like the left
one.

Fixes #388
gl_video: use hwdec download_image call only if hwdec is active

The hwdec driver can be loaded, even if it's not used (e.g. when playing
a file with no hardware decoding after one with it enabled).

Also, check whether dlimage is NULL. Since this will do call into the
native hwdec API, there's a chance a driver could fail doing this, it's
better to check the return value, even if this case currently can't
happen.
manpage: mention that vda can work with vo_opengl

Also, with the addition of VDA, all hardware decoding backends work with
vo_opengl now.
gl_video: change internal API for hwdec mp_image download

Previous API worked under the assumption that download_image is always called
after map_image. In practice this is true, but it's better to have a much
generic API that doesn't depend on the order in which the functions are called.
vf_screenshot: remove unnecessary code
video: remove VFCAP_POSTPROC

This wasn't used anymore, not even until recently.
vf_lavfi: export a wrapper function

This will allow old filter to run libavfilter instead by calling
vf_lw_set_graph(), which turns the filter into a wrapper, using a given
libavfilter graph.

Later commits use that to automatically "reroute" a bunch of filters to
libavfilter. We want to get rid of the old MPlayer filter code, because
it's bad an unmaintained, but we still don't want to force everyone to
use vf_lavfi, so this solution will do for a while.
vf_yadif: change options, reroute to vf_lavfi

Also remove the ability to disable deinterlacing at runtime. You can
still disable deinterlacing at runtime by using the ``D`` key and its
automatical filter insertion/removal.
vf_delogo: reroute to vf_lavfi

The ``file`` suboption is unsupported on lavfi.
vf_gradfun: reroute to vf_lavfi

Also update the manpage. We changed defaults and added a suboption quite
some time ago, and we forgot to update the manpage at all.
vf_hqdn3d: change options, reroute to vf_lavfi
vf_stereo3d: reroute to vf_lavfi