Basic "support" for random play PGCs
Some discs use a PGC with playback_mode set to random as part of their protection: if you randomly select the next program/cell to play, then you end up in the proper menu. If you always choose the first one, you end up jumping to a title containing: 1) bad MPEG data that causes ffmpeg to crash with an FPE; 2) if you survive that then you walk into bad sectors which cause the drive to repeatedly spin down and up again, and is a pain to get out of.
This is not full random play support, we only choose the first program randomly. (We could also choose the next one randomly at the end of each cell, but this is sufficient for those discs.)
It also doesn't support shuffle play (random but without repetition), which would require storing extra state so we play each program once and only once.
(This requires libdvdread#5 (closed) to be applied first, as the field ordering is broken in current libdvdread and pg_playback_mode incorrectly reads as 0.)