Only Build Display Items for Background if Really Necessary

[pyfisch]

There are at least two cases where Display Items for backgrounds are emitted when they are not needed for rendering:

1. Lower background layer is completely covered by opaque upper layer.
2. A background image or gradient where at least one size is equal zero.

I am not sure what the performance impact of both optimizations would be. While it is very common that a gradient and a solid fallback color are specified (case 1.) there are some optimizations on the webrender side to avoid actual painting by the GPU.