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FFmpeg: Improve proxy building performance

Use h264 codec for output. This codec produces smaller files, can be
multithreaded and decodes even faster than MJPEG.

Quality setting 0-100 corresponds to "Lowest Quality" to
"Perceptually Lossless" in Blender's h264 encoding presets.

All available cores are used for decoding.
Same goes for decoding but only for codecs that supports this
(h264, vp9 seems to support this option out of th box as well).
Other decoders can probably be optimized in similar way, but threaded
encoding provides significant boost already.

I have tested variety of codecs, and all were transcoded properly.

Reviewed By: sergey, fsiddi

Differential Revision: https://developer.blender.org/D10731