base

Composite layers as regions.

This change detects what area of a layer was drawn into and generates a mesh
to match this area exactly. This can be used to avoid blending empty pixels
when the layer is composited.
This change also adds proper layers support to lines rendering and implements
layers composition in a more readable way.

Change-Id: I4a5588b98b19bd66891ebdc39631b193c5e31999