avcodec: only use thread-safe buffers when not using a threade decoder
Split thread-safe buffers and syncing to the processing thread.

This allows a decoder to have thread-safe buffers and gain some advantages
from this, while still syncing to the main thread for more consistent
behaviour.
Remove MT Filtering thread

While this method does slightly increase performance, it does this at the
cost of smooth playback. So instead we accept a small performance penality
in benchmarks (should not affect real-world situations), but guarantee
much smoother playback instead.
Remove superflous condition
avcodec: don't sync to the process thread, even if multi-threaded
Update ffmpeg for two mkv crashes
Fix media type handling when switching video streams
Update changelog and version