renderer: merge similar planes before heavy operations

This makes most types of planar operations faster, especially scaling, debanding, AV1 grain application and other plane shaders. Care needs to be taken to avoid doing this unnecessarily though, to avoid an FBO roundtrip when there's no point.

Note that we try re-using the same plane texture format if possible, and only pick as many components for the FBO as we deem necessary. This cuts down on memory bandwidth.

Closes #26 (closed)