BUG: Fix GetProcAddress call when OSMesa is being used.

When OSMesa was used (and VTK was built without X), we ended up not loading any
of the extensions since GetProcAddress() didn't use the equivalent call for
OSMesa i.e. OSMesaGetProcAddress(). This patch addresses that issue.

Change-Id: Ia4e83f6884bd6e178f160934ff749cff4ef1e788
Merge topic 'osmesa_extensions' into master

a142841a BUG: Fix GetProcAddress call when OSMesa is being used.