Steady frames

Fixes:

• Scrubbing the timeline could momentarily flash black over video while frames decoded. The preview now always holds the last frame until the next one is ready — and exports can no longer emit a black frame in those spots either.
• Changing a color sometimes added a duplicate entry to your undo history, making undo feel like it did nothing. Every color edit is now exactly one undo step, and picking the same color again adds none.