Blender Git Loki

FFmpeg: Update proxy settings

Changes in rBce649c73446e, affected established proxy codec preset.
Presets were not working and all presets were similar to `veryfast`.
Tunes are now working too, so `fastdecode` tune can be used. I have
measured little improvement, but I tested this only on 2 machines and
I have been informed that `fastdecode` tune does influence decoding
performance for some users.

Change preset from `slow` to `veryfast` and add tune `fastdecode`

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D11454