Improve gate animation behavior

[historic_bruno]

Requires a few general animation engine changes (see comments from #619):

• Allow skipping to certain time in the animation (also useful for sails)
• Allow reversing animation direction (useful for gates and sails)
• Serialize current animation time so animations don't replay when loading a saved game (especially noticeable with run-once animations, like gate opening/closing)

Somewhat related to #1195.