ad_lavc: allow skip samples amount to be larger than 1 packet

Apparently we actually need this. At least the following commit would
break without this.
demux_mkv: implement audio skipping/trimming

This mechanism was introduced for Opus, and allows correct skipping of
"preroll" data, as well as discarding trailing audio if the file's
length isn't a multiple of the audio frame size.

Not sure how to handle seeking. I don't understand the purpose of the
SeekPreRoll element.

This was tested with correctness_trimming_nobeeps.opus, remuxed to mka
with mkvmerge v7.2.0. It seems to be correct, although the reported file
duration is incorrect (maybe a mkvmerge issue).
demux_mkv: remove minor code duplication
demux: don't consider stream EOF an underrun

In this case, we didn't find any new packets for this stream, even
though we've read ahead as much as possible. (If reading ahead in this
case, the "Too many packets in the demuxer packet queues" error is
normally printed.)

If we do consider this an underrun, handle_pause_on_low_cache() will
pause and show the "buffering" state, which is not useful.

Could also happen on very bad interleaving.
demux: fix PTS comparison

This was relying on the fact that timestamps will always be numerically
larger than MP_NOPTS_VALUE, but the trick didn't actually work for
MP_PTS_MIN. Be a bit more sincere, and don't rely on this anymore. This
fixes the comparison, and avoids the readahead amount displaying as
"???" in some situations (since one of the values was NOPTS).
demux: don't account known range for streams that are EOF

This influences the demuxer readahead display. If a stream has reached
EOF, we want to ignore it for the purpose of this calculation.

Note that if a stream contains no packets, it still should cause the
value 0s to be displayed (unless it's EOF), because that's just the
actual situation.
demux: unbreak build with Libav

....
x11: rely on the Atom cache

XInternAtom() has a 64 entry hash table to avoid network accesses. Rely
on this cache, instead of caching these manually.
vo_opengl: minimal EGL on X11 support

Pretty useless and only good for testing.

Does not include any form of GLES support.
player: fix --secondary-sid

Use the "default" selection for the ff-index, not the "no" selection.

Broken by commit f0f83ff.
Fixes #1243.
video: get hwaccel flag from pixdesc
video: refuse to allocate image data for hwaccel formats

Makes no sense.
video: handle endian detection in a more generic way

FFmpeg has only a AV_PIX_FMT_FLAG_BE flag, not a LE one, which causes
problems for us: we want to have the LE flag too, so code can actually
detect whether a format is non-native endian. Basically, we want to
reconstruct the LE/BE suffix all AV_PIX_FMT_*s have.

Doing this is hard due to the (messed up) way AVPixFmtDescriptor works.
The worst is AV_PIX_FMT_RGB444: this group of formats describe an
endian-independent access (since no component actually spans 2 bytes,
you only need byte accesses with a fixed offset), so we have to go
through some pain.
video: passthrough unknown AVPixelFormats

This is a rather radical change: instead of maintaining a whitelist of
FFmpeg formats we support, we automatically support all formats.

In general, a format which doesn't have an explicit IMGFMT_* name will
be converted to a known format through libswscale, or will be handled
by code which can treat pixel formats in a generic way using the pixel
format description, like vo_opengl.

AV_PIX_FMT_UYYVYY411 is a special-case. It's packed YUV with chroma
subsampling by 4 in both directions. Its component order is documented
as "Cb Y0 Y1 Cr Y2 Y3", meaning there's one UV sample for 4 Y samples.
This means each pixel uses 1.5 bytes (4 pixels have 1 UV sample, so
4 bytes + 2 bytes). FFmpeg can actually handle this format with its
generic mechanism in an extremely awkward way, but it doesn't work for
us. Blacklist it, and hope no similar formats will be added in the
future.

Currently, the AV_PIX_FMT_*s allowed are limited to a numeric value of
500. More is not allowed, and there are some fixed size arrays that need
to contain any possible format (look for IMGFMT_END dependencies).

We could have this simpler by replacing IMGFMT_* with AV_PIX_FMT_*
through the whole codebase. But for now, this is better, because we
can compensate for formats missing in Libav or older FFmpeg versions,
like AV_PIX_FMT_RGB0 and others.
video: add image format test program
video: remove aliases for some rarely referenced image formats

These formats are still supported; you just can't reference them via a
defined constants directly. They are now handled via the generic
passthrough.

(If you want to use such a format, you either have to add the entry
back, or use AV_PIX_FMT_* directly.)
vo_wayland: don't use endian-specific defines

Instead, use the native-endian alias, and switch the wayland format
depending on the target platform's endian.

This drops support for swapped-endian formats, but I think that is ok.
Not only are the affected formats rather ancient and backwards, but
using swapped formats probably does not make any sense for performance
either.

Untested.
video: remove swapped-endian image format aliases

Like the previous commit, this removes names only, not actual support
for these formats.
demux: report 0s readahead time as fallback in some situations

If no packets are queued, the readahead time is obviously 0.

If the end time is smaller than the start time, the problem is probably
that audio and video start at slightly different times - report 0 in
this case too.

Do this because seeing "???" as readahead time is a bit annoying.
command: use playback time as reference for sub_seek

update_subtitle() already uees playback_pts to make subtitles work
better in no-audio mode. Using get_current_time() usually gets
playback_pts, but also has the advantage that it will use the seek
target time during seeks. This will result in multiple sub_seek commands
doing the right thing (at least as long as they're far enough apart so
that seeking is actually initiated when the second command is run).