[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] Update ffmpeg for "avformat/mpegts: check avio_tell() return code before using it".

This commit fixes MPC-HC ticket #3466.
[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.