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
