compositor/glvideomixer: Add support for cropping and aspect-ratio preserving scaling

Submitted by Sebastian Dröge @slomo

Link to original bug (#756276)

Description

We have width/height properties for scaling, but additional properties for cropping would be useful to have.

On top of that, having the width/height properties to automatically preserve the aspect-ratio would be great. E.g. by only setting one of width or height, and having the other be calculated by compositor/glvideomixer