save memory during render and cache render results.
Implementation notes:
In the render engine API it's now possible to get the render result for
one render layer only, and retrieve the expected tile size in case save
buffers is used. This is needed because EXR expects tiles with particular
size and coordinates.
The EXR temporary files are now also separated per layer, since Cycles
can't give the full render result for all render layers, and EXR doesn't
support writing parts of tiles.
In Cycles internally the handling of render buffers and multi GPU
rendering in particular changed quite a bit, and could use a bit more
refactoring to make things more consistent and simple.
* 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.
