Remove obsolete dxva2api header

Any supported build environment includes a more recent dxva2api.h, and this header was never used anymore.
Update ffmpeg
Remove unused define
cuvid: Use the shell window instead of creating our own
cuvid: clear function pointers after freeing the lib
Cleanup pthreads lockmgr
Respect channel expansion settings when creating the initial mediatype
LAV Splitter: Fix the filtering of non-forced PGS subtitles.

PGS segments with just a header were mistakenly ignored. This included "end of display" segments so it could result in no subtitle being displayed at all.
[MPC-HC] Use our own ffmpeg clone repository.

WARNING: This should be the first custom commit in our fork and its title should not be modified.
[MPC-HC] LAV Splitter: Exhibits MKV attachments through IDSMResourceBag interface.
[MPC-HC] Tray icon: Add an optional custom callback for "show property page" events.
[MPC-HC] Fix the include directories for FFmpeg's out-of-source-tree build.
[MPC-HC] Version number: Add a "git describe based" commit number to identify the vanilla LAV Filters we are based on.

This way if we distribute LAV Filters version X.Y.Z.N, it means that our custom build is based on:
 - the stable version X.Y.Z,
 - with N official patches (which will be part of next official LAV Filters build),
 - and some custom non-official patches which are here to improve the integration in MPC-HC.