sub: handle vobsub-in-mp4

The mplayer decoder (spudec.c) actually handled this. There was explicit
code for binary palettes (16 32 bit values), and the subtitle resolution
was handled by video resolution coincidentally matching the subtitle
resolution.

Whoever puts vobsub into mp4 should be punished.

Fixes the sample gundam_sample.mp4, closes github issue #547.