New function to draw offscreen, and related API changes

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

doc/python_api/sphinx_doc_gen.py

@@ -261,6 +261,7 @@ else:
         "bpy.utils.previews",
         "bpy_extras",
         "gpu",
+        "gpu.offscreen",
         "mathutils",
         "mathutils.geometry",
         "mathutils.bvhtree",
@@ -1659,7 +1660,9 @@ def write_rst_contents(basepath):
         # mathutils
         "mathutils", "mathutils.geometry", "mathutils.bvhtree", "mathutils.kdtree", "mathutils.noise",
         # misc
-        "freestyle", "bgl", "blf", "gpu", "aud", "bpy_extras",
+        "freestyle", "bgl", "blf",
+        "gpu", "gpu.offscreen",
+        "aud", "bpy_extras",
         # bmesh, submodules are in own page
         "bmesh",
         )
@@ -1799,6 +1802,7 @@ def write_rst_importable_modules(basepath):
         # C_modules
         "aud"                  : "Audio System",
         "blf"                  : "Font Drawing",
+        "gpu.offscreen"        : "GPU Off-Screen Buffer",
         "bmesh"                : "BMesh Module",
         "bmesh.types"          : "BMesh Types",
         "bmesh.utils"          : "BMesh Utilities",