The disc VM advances when the stream is read, so in nav mode the cache
must not run ahead of what the user is seeing. Never read ahead (a
looping menu is endless). Never satisfy seeks from the cache either.
This applies to the whole disc session, not only while a menu is on
screen. The VM can jump at any time (BD-J timers, end of title), so
the cache must never run ahead of playback.
Activations still must be able to drive the VM while nothing is
consumed. demux_drive_nav() forces a single read for them.
Kacper Michajłow
f0bc30aaf1