http://wiki.blender.org/index.php/Dev:2.6/Source/Render/RenderEngineAPI
* This adds a Rendered draw type in the 3D view, only available when
the render engine implements the view_draw callback.
* 3D view now stores a pointer to a RenderEngine.
* view_draw() callback will do OpenGL drawing instead of the viewport.
* view_update() callback is called after depsgraph updates.