This is a first step towards proper depsgraph "ownership", where
we would allow scene to be in multiple states dependent on active
workspace or scene layer.
This commit introduces a basic API to get proper dependency graph
for a given scene layer. It also renames scene->depsgraph to
depsgraph_legacy, so it's easier to search0-n-replace in the future.
This expose the capability of handling offscreen drawing. The initial
support lays the barebones for addons to work with framebuffer objects
and implement 3d viewport offscreen drawing. This can be used by script
writers to make fisheye lens preview, head mounted display support, ...
The complete documentation is here: http://www.blender.org/api/blender_python_api_2_76_1/gpu.offscreen.html
Review and many changes by Campbell Barton (thank you :)
https://developer.blender.org/D1533
