GRIM/Movie: Enable seeking at savegame restore.
GRIM/Movie: Lock the frame mutex when restoring a savegame.

Fix a race condition which could make it seek to time 0 instead of the time
saved in the savegame.
GRIM/Movie: Fix smush looping.
GRIM/Movie: Init the frames only when needed.
GRIM:/Movie: Remove two warnings.
GRIM/Movies: Cleanup outdated comments.
Merge pull request #709 from giucam/smushseek

Enable seeking smush movies when restoring a savegame