Output stream filtering

In this release output image processing may be done in one of two paths:

• when there is only need to change pixel format, it's done with fast, multi-threaded way using swscale library,
• when other processing is required, it may be done providing <filter> value in casparcg.config. This may be significantly slower, however it may also be multi-threaded, if FFMpeg itself allow it.

Also moved pix_fmt to <options>.