New scenegraph renderer and atlas textures.

The renderer tries to batch primitives together where possible, isolate
non-changing subparts of the scene from changing subparts and retain
vertexdata on the GPU as much as possible. Atlas textures are crucial
in enabling batching.

The renderer and atlas texture are described in detail in the doc page
"Qt Quick Scene Graph Renderer".

Change-Id: Ia476c7f0f42e1fc57a2cef528e93ee88cf8f7055
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>