This is a rollup of the following patches
part 0: Add a dynamic cast to ColorLayer*
part 1: Add a Composer2D interface to enable implementations to more efficiently compose layer trees
part 2: Let widgets expose Composer2Ds, if they have them
part 3: Expose a layers ogl "friend" API that Composer2D will consume
part 4: Hook Composer2D into the LayerManagerOGL rendering pipeline
part 5: Implement all the goop to let widget/gonk use a Composer2D (HwcComposer2D)