Bug 1064479. Redo how screen rotation is implemented on B2G/Layers. r=nical,mwoodrow

Currently we use a world transform on the LayerManager. This gets passed to the
screen render target which adjusts the gl transform to accomplish the rotation.
This causes a lot of the system to have to know about the transform.

Instead we can just bake the transform into the root layer's shadow transform.
Everything now mostly just works.

--HG--
extra : rebase_source : e0e170d191eef5887a2e6b17f71ac79037033006