demux_mkv: never force output sample rate

Matroska has an output sample rate (OutputSamplingFrequency), which in
theory should be forced instead of whatever the decoder outputs. But it
appears no software (other than mplayer2 and mpv until now) actually
respects this. Even worse, there were broken files around, which played
correctly with (in theory) broken software, but not mplayer2/mpv. Hacks
were added to our code to play these files correctly, but they didn't
catch all cases.

Simplify this by doing what everyone else does, and always use the
decoder's sample rate instead. In particular, we try to handle all
sample rate issues like libavformat's Matroska demuxer does.
demux_mkv: ignore DefaultDuration in some cases

This fixes playback of the sample linked by FFmpeg ticket 2508. The fix
follows ffmpeg commit 6158a3b (although it's not exactly the same).

The problem here is that the file contains an apparently non-sense
DefaultDuration value. DefaultDuration for audio tracks is used to
derive PTS values for packets with no timestamps, like they can happen
with frames inside a laced block. So the first packet of a SimpleBlock
will have a correct PTS, while the PTS values of the following packets
are calculated using DefaultDuration, and thus are broken.

This leads to seemingly ok playback, but broken A/V sync. Not using the
DefaultDuration value will leave the PTS values of these packets unset,
and the audio decoder can derive them from the output instead.

The fix more or less uses a heuristic to detect the broken case: if the
sample rate is 8 KHz (Matroska default, can assume unset), and the codec
is AC3 (as the broken file did), don't use it. I'm not sure why this
should be done only for AC3, maybe the muxing application (mkvmerge
v4.9.1) has known issues with AC3. AC3 also doesn't support 8 KHz as
sample rate natively.

(By the way, I'm not sure why we should honor the DefaultDuration at all
for audio. It doesn't seem to be needed. You can't seek to these frames,
and decoders should always be able to produce perfect PTS values by
adding the duration of the decoded audio to the first PTS.)
sd_ass: fix font aspect ratio with non-ASS subs

For subtitles that were not ASS, the subtitle font aspect was always set
to that of the video, the exact opposite of how it was supposed to
behave.
sd_lavc: don't stretch DVD subtitles to video aspect

I'm not sure what's correct: stretching the DVD subtitles from storage
aspect ratio to video display aspect ratio, or displaying subtitles
using 1:1 PAR. Until now, DVD subtitles (as well as all other bitmap
subtitles) were always stretched to the video. There are good arguments
why this would be the correct behavior: DVDs were made for playback on
TV, which display anamorphic video by adjusting the horizontal refresh
rate, and thus wouldn't even be capable of DVD subtitles with square PAR
(other than resampling the subtitles additionally).

However, I haven't seen a sample yet where subtitles do _not_ look
stretched using this method. Rendering them at 1:1 PAR looks better.
Technically, we render them at display PAR (and not 1:1 PAR). Do this in
a way so that the subtitle area is always inside of the video frame if
display and video aspect ratios mismatch.

For DVB subtitles, the old method looks more correct, so this is special
cased to DVD subtitles.

I might revert this commit if it turns out that it's an disimprovement.
core: move video refresh function to mplayer.c
video: redo how colorspaces are handled

Instead of handling colorspaces with VFCTRLs/VOCTRLs, make them part of
the normal video format negotiation. The colorspace is passed down like
other video params with config/reconfig calls.

Forcing colorspaces (via the --colormatrix options and properties) is
handled differently too: if it's changed, completely reinit the video
chain. This is slower and requires a precise seek to the same position
to perform an update, but it's simpler and less bug-prone. Considering
switching the colorspace at runtime by user-interaction is a rather
obscure feature, this is a good change.

The colorspace VFCTRLs and VOCTRLs are still kept. The VOs rely on it,
and would have to be changed to get rid of them. We'll do that later,
and convert them incrementally instead of in one go.

Note that controlling the output range now always works on VO level.
Basically, this means you can't get vf_scale to output full-range YUV
for whatever reason. If that is really wanted, it should be a vf_scale
option. the previous behavior didn't make too much sense anyway.

This commit fixes a few bugs (such as playing RGB video and converting
that to YUV with vf_scale - a recent commit broke this and forced the
VO to display YUV as RGB if possible), and might introduce some new
ones.
Use /dev/cd0 as default cdrom device on FreeBSD