[Meta] Enable more elements to be directly composited

• Component: Layout and Rendering
• Status: NEW ---

[Noam Rosenthal]

When using accelerated compositing, right now we directly composite images, and in some cases background. For any other type of content, we have to create a backing store and render into it in software.
This creates:
* memory overhead for maintaining those backing stores
* CPU overhead in rendering those with software
* Overhead when uploading those backings to a texture.

The following elements could potentially be directly composited, and thus benefit from faster rendering and lower memory footprint:
* Background color
* Some background images
* Gradients
* Some borders/outline
* Border radius
* Nine-piece images
* Crossfade
* Box shadow

Also once more elements are directly composited, playing with the layer configuration in RenderLayerCompositor might be possible as an optimization.

Since making something composited that wasn't composited before is a delicate change with many possible implications, the idea is to do this slowly and carefully.
This bug is here to track those changes and provide some context for them.