[WIP] Add videoscale and ffmpegcolorspace elements.

This is preliminary work for the PiP plug-in stabilization based on some feedback from #gstreamer, but I'd like some feedback.

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Commit Message:

Some cameras do not support resizing to certain dimensions and specific
pixel formats. The videoscale and ffmpegcolorspace elements allow us to
scale to any dimension and convert to a specified pixel format.