demux_mkv: support svq3

The most awesome codec, not.

The actual code for svq3 is actually just the part that checks for
MKV_V_QUICKTIME (no other QT-muxed codecs are supported). The rest is
minor refactoring, that actually improves the code in general.

This is just enough to support the 2 svq3-in-mkv sample files I have.