Improve the speed of subtitles toggling (on/off) and switching.

Avoid reloading the subresync bar if the same subtitles were already loaded.

This commit fixes #3650.
CFilterMapper2 hooks: Delay error reporting until the hooks are really needed.
Cosmetics and documentation for the hooks.
Only report error for important hooks and try to run anyway.

Mostly DVD playback is impacted if some hook fails but the user might not plan to play DVD and it might work anyway.
Use conversion constructors to work-around confusing AStyle styling.
Use a bitcheck instead of casting (WIP).

Fixes CID #965944.