wayland: add wayland support

All wayland only specific routines are placed in wayland_common.
This makes it easier to write other video outputs.

The EGL specific parts, as well as opengl context creation, are in gl_common.

This backend works for:
    * opengl-old
    * opengl
    * opengl-hq

To use it just specify the opengl backend
    --vo=opengl:backend=wayland
or disable the x11 build.

Don't forget to set EGL_PLATFORM to wayland.

Co-Author: Scott Moreau
(Sorry I lost the old commit history due to the file structure changes)
vo_sdl, vo_xv: remove redundant/useless VOCTRL_PAUSE usage
core: simplify OSD capability handling, remove VFCAP_OSD

VFCAP_OSD was used to determine at runtime whether the VO supports OSD
rendering. This was mostly unused. vo_direct3d had an option to disable
OSD (was supposed to allow to force auto-insertion of vf_ass, but we
removed that anyway). vo_opengl_old could disable OSD rendering when a
very old OpenGL version was detected, and had an option to explicitly
disable it as well.

Remove VFCAP_OSD from everything (and some associated logic). Now the
vo_driver.draw_osd callback can be set to NULL to indicate missing OSD
support (important so that vo_null etc. don't single-step on OSD
redraw), and if OSD support depends on runtime support, the VO's
draw_osd should just do nothing if OSD is not available.

Also, do not access vo->want_redraw directly. Change the want_redraw
reset logic for this purpose, too. (Probably unneeded, vo_flip_page
resets it already.)
vf_vo: remove pointless NULL checks

The filter refuses to initialize if the video_out is NULL.
core: move X11 specific code to x11_common.c

Nothing changes, because vo_check_events() is called at the same place
anyway.
m_options: more typesafety

Change the option definition macros so that they cause compiler warnings
if the type of the referenced option struct member doesn't match the
type implied by the macro. The compiler warning printed isn't very
telling, but it's better than silently invoking undefined behavior by
violating the C strict aliasing rules.

Also fix some minor cases that violate the type rules. For the option
"no-aspect" we have to add a new option type to handle it properly.

Some option types are hard to check, so we don't in these cases.
m_option: reformat option macro definitions

Slightly more bearable, but it's still incredibly ugly.
m_option: don't define OPT_BASE_STRUCT by default

OPT_BASE_STRUCT defines which struct the OPT_ macros (like OPT_INT etc.)
reference implicitly, since these macros take struct member names but no
struct type. Normally, only cfg-mplayer.h should need this, and other
places shouldn't be bothered with having to #undef it.

(Some files, like demux_lavf.c, still store their options in MPOpts. In
the long term, this should be removed, and handled like e.g. with VO
suboptions instead.)
vf_flip: move flipping code to mp_image.c
demux_lavf: fix percentage display with some file formats

AVFormatContext.start_time is sometimes AV_NOPTS_VALUE, such as when
playing FLAC files. (For most other file formats it's set to 0, even if
the format doesn't support arbitrary start times.)
core: fix SEEK_FACTOR

Emulate percentage-seeks (SEEK_FACTOR) as normal time-seeks if possible.
This fixes some issues with (let's call it) low quality implementations
of SEEK_FACTOR (e.g. demux_mkv basically interprets this as byte-seek,
and also seeking to 99.9% makes it seek back to the start).

For weird MPEG formats the demuxer level SEEK_FACTOR is still used.
These formats, which can have timestamp resets, are identified by
setting demuxer->ts_resets_possible to true.

Also, have get_current_pos_ratio() follow the same rules, and calculate
the percentage position with the file position if timestamp resets are
possible.

This actually fixes percentage-seeks in .ts files with demux_lavf.c.
This kind of seek is not really used now, but it will be more important
when we add a progress bar.

Note: seeking in chained ogg files is still completely broken. The main
issue is that ffmpeg doesn't provide a sane API for dealing with
timestamp resets, and trying to do byte seeks with ogg confuses demuxer
and decoder (or something like this) and just does random things.
(Tested with two concatenated flac-in-ogg files).
manpage: remove reference to subtitle alignment
input.conf: change default bindings of 5/6 from hue to gamma

I doubt anyone needs to adjust hue on a frequent basis, and gamma is
much more useful.

Suggestions for more radical changes of key bindings are welcome
(there's a lot of useless crap mapped).
Merge branch 'master' into osdbar_experiment