* Scene.use_shading_nodes property to check if RenderEngine is using new shading
nodes system, and RenderEngine.bl_use_shading_nodes to set this.
* Add mechanism for tagging nodes as being compatible with the old/new system.
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.
http://wiki.blender.org/index.php/Dev:2.6/Source/Render/RenderEngineAPI
* RenderEngine is now a persistent python object that exists and retains
properties as long as a frame is being rendered. This is mostly useful now
that more than one callback will be added.
* Added update() callback that should ideally be used to export the scene,
leaving only the rendering to the render() callback. This is not required to
be used at this point, but separating this will make things more thread safe
later on.
* Added tag_redraw() and tag_update() functions that will be used for viewport
rendering.
* Internal change: status text is now retained after update_status calls.
