2.5: Render

2.5: Render

This adds a RenderEngine type to RNA, which can be subclassed
in python (c++ will follow once we support subclassing there).
It's very basic, but plugs into the pipeline nicely. Two example
scripts:

http://www.pasteall.org/6635/python
http://www.pasteall.org/6636/python

Issues:
* Render runs in a separate thread, and there is unrestricted
  access, so it's possible to crash blender with unsafe access.
* Save buffers and full sample are not supported yet.