mplayer: fix volume setting with --gapless-audio

Playing something with "mpv f1.mkv f2.mkv --gapless-audio --volume=20"
caused the volume to be reset when playing a new file. Normally, the
volume should not be reset (unless explicitly requested with per-file
options), and without either --gapless-audio or --volume it works as
expected.

The underlying problem is that volume was saved only when the AO was
uninitialized, and also the volume was always set when starting a file.
Fix this by saving the volume when playback ends, and when the audio
is reinitialized. To make sure the volume is never restored twice or
saved in the wrong situation, introduce INITIALIZED_VOL.

Also note that this volume saving and restoring only happens if the
--volume option is used. mixer.c does its own bookkeeping of volume.
The main reason for this is that the volume option could be reset by
per-file options (see manpage), and mixer.c doesn't know anything
about this stuff. This is probably dumb, and maybe some things could
be simplified. But for now this will work.
options: remove some questionable -lavdopts suboptions

Most of these are rather questionable, the rest you rarely need to set
manually. You still can set all of them with -lavdopts-o (because
libavcodec has AVOptions for them).
demux_asf: fix after commit 5165e19

This demuxer reallocated packets on its own, instead of using the
demux.c functions, which clashed with a recent change.
core: avoid deselecting and reselecting stream needlessly

The core deselected all streams on initialization, and then selected the
streams it actually wanted. This was no problem for
demux_mkv/demux_lavf, but old demuxers (like demux_asf) could lose some
packets. The problem is that these demuxers can buffer some data on
initialization, which then is flushed on track switching. Fix this by
explicitly avoiding deselecting a wanted stream.
gl_video: fix some dithering bugs

The internal texture format GL_RED is typically 8 bit, which is clearly
not good enough for the new dither matrix. The idea was to use a float
texture format, but this was somehow "forgotten". Use GL_R16, since
16 bit textures are more robust, and provide more precision for the
same memory usage.

Change how the offset for centering the dither matrix is applied. This
is needed for making it possible to round up values to the target depth.
Before this commit, this changed the output even if the input was exact
and input and output depth were the same, which is not really what you
want. Now it doesn't do that anymore.
gl_video: change a GLSL statement

This might be better with dumb shader compilers, which won't vectorize
this to a single vector-division, assuming the hardware does have such
an instruction. Affects "bicubic_fast" scale mode only.
gl_video: add some debug code for testing texture depth

This probes and prints the depth of some texture formats with the help
of a FBO. By default it tests the format used for scaling, as well as
the format used for dithering and the 3D LUT (if any of these are
enabled).

The output is visible only with -v. Some representative values are
probed, and the difference of input and output value is printed as hex-
float. Hex-floats are used because they make the implied precision more
obvious. Originally I wanted to do some more sophisticated guessing of
the implied depth/precision for more user-friendly reporting, but then
I decided that printing raw data is better for debugging, especially if
things go wrong.

This does not try to disable any functionality and does not print any
warnings if the depth is lower than what it should be.
gl_common: fix invalid alignment

0 is invalid. The intention of the code turning off any additional
alignment, so we need 1.

Change a comment: obviously we don't try to set alignment parameters
etc.to handle stride correctly, and instead do everything by row.