CPPageSubtitles: Various cosmetics.

Reduce the visibility of variables and functions, remove unused variables and functions, use better name.
ISR: Animated subtitles: Add the ability to control the animation rate.
Subtitle queue: Correctly handle the cases when rendering is paused.

In such cases, try to block until the subpic is ready since the frame won't be refreshed and the subtitle wouldn't be displayed after seek or resize if the queue is too slow.
Subtitle queue: Add the ability to share the lock on the subtitle provider.

The queue can access the subtitle provider concurrently in a read-only manner that allows sharing the same lock to improve the performance.

That is if the queue already owns the lock, it doesn't have to lock again, just ensure that the lock won't be released too early.