Fix: DVD subtitles were always re-enabled when clicking on a menu entry related to external subtitles.

This commit fixes #4141.
Rename "Enabled" menu entry to "Enable DVD subtitles" for clarity.

This commit fixes #4141.
Fix: Subtitles could be partially rendered when using no subtitle queue.

The previously allocated subpic was reused without checking if it was big enough.
SubPicAllocator: Add a thread-safe way to free the static subpicture.

This is for later use but the static subpicture was currently freed without any locking when changing the DirectX device and I am not quite sure this was safe at all.
ISR: Text subtitles were incorrectly displayed after switching from PGS/DVB subtitles.
Add an implementation of CStdioFile that supports file bigger than 2GB.
Use CStdioFile64 to open subtitles files and remove the try/catch that used to hide the issue.
Prevent the low-merit fall-back on internal LAV Splitter to demux raw subtitle formats.

This commit fixes #3544.
Cosmetics: Factor some duplicated code.