PGS subtitles: Update RemoveOldSegments to account for unknown end time.

This was broken after d8deeda49e270d02e9eae795265494184f409b29, some subtitles could be wrongly removed.
Revert "Fix rare black screen issue with madvr"

This reverts commit 455619eadd8c157f977041bdbad4522c47166eb3.

Not reproducible with latest mpc-hc and madvr.
Add wrapper for dynamic winapi function loading
MainFrame: allow unelevated taskbar messages

Fixes #4181