Blender Git Loki

FFmpeg: Improve multi-threading settings

Allow use all system threads for frame encoding/decoding. This is very
straightforward: the value of zero basically disables threading.

Change threading policy to slice when decoding frames. The reason for
this is because decoding happens frame-by-frame, so inter-frame threading
policy will not bring any speedup.

The change for threading policy to slice is less obvious and is based on
benchmark of the demo files from T78986. This gives best performance so
far.

Rendering the following file went down from 190sec down to 160sec.

https://storage.googleapis.com/institute-storage/vse_simplified_example.zip

This change makes both reading and writing faster. The animation render
is just easiest to get actual time metrics.

Differential Revision: https://developer.blender.org/D8627