WINTERMUTE: Fix bug that prevented the opaque and binary blit speedups from working.
WINTERMUTE: Special-case FMV-handling to not fill the screen with background color.

If we have only one thing being drawn, and that is opaque, we can
skip filling the render surface with background color.
This shaves another few wasted cycles of the FMV playback. (Since we
now don’t have to write the entire render surface TWICE).

This reduces the time spent in drawTickets() to ~60% of what it was before.