Remove the unused cpu_painting field of CompositorData
Use 'background_color' consistently over 'unrendered_color'
Fix indentation
Simplify CompositorData creation

We already have a layer_properties struct, so we should always use it
instead of passing the information in it out-of-band.
Always create CompositorData with a new layer

Since it makes no sense to create one without the other, the public
interface shouldn't let you do anything else.