Blender Git Loki

Improve proxy building performance

There are minimal changes to current code:
- Use h264 codec for output
- Specify number of threads for encoding to be same as system thread count
- Specify same nuber of threads for decoding. This may work only with some codecs(only h264 tested so far), but performance gain in encoding improves overall performance by big margin still. I have tested variety of codecs, and all were transcoded properly.

This is much simpler and straightforward patch than previous two, and this was in fact first thing I have tried to do in the beginning, but there was no improvement unless I have removed following lines:
```
rv->c->thread_count = BLI_system_thread_count();
rv->c->thread_type = FF_THREAD_SLICE;
```

I am not even sure how I found that these two lines were problematic.

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