Update to the upstream GLEW 1.11.0 release

Preserved upstream formatting etc for easier updates in the future.

Change-Id: I57bfc51c75c02ade4194b108470096078129e24d
Added logic for OSMesa contexts

This will enable VTK's GLEW to make use of OSMesa OpenGL contexts,
unfortunately support for this has yet to be merged upstream.

Change-Id: I6ead4a9f4ddd7276fdad806af6093343145307dd
Add support for OSMesa builds of OpenGL2

This commit makes some changes in Mesa's logic so that it can use
OSMesa's OpenGL context, and load extensions in OSMesa. This works
on Linux, but will need more work to get it working on Windows
and/or Mac OS X. Pulled out the OpenGL Mesa logic so that both GLEW
and vtkRenderingOpenGL2 can use it (and ensure it is consistent).

Change-Id: Ibdf6715f950929949f5055b3ddbd38b09d9b84d6
Merge remote-tracking branch 'origin/master' into glew

Conflicts:
	Rendering/OpenGL2/CMakeLists.txt

Change-Id: I984aa8000e0248fbc0c9c4870d3b35fbb2003df9
Merge topic 'glew' into master

207c2594 Merge remote-tracking branch 'origin/master' into glew
c0dee401 Add support for OSMesa builds of OpenGL2
1b2d22f3 Added logic for OSMesa contexts
e124dd9d Update to the upstream GLEW 1.11.0 release