Blender Git Loki

RenderEngine: steps towards implementation of this proposal:
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.