Added support to vtkOpenGLExtensionManager for fine grained
OpenGL driver version detection. Added a flag to enable/disable
features with known driver specific bugs for testing new
driver releases. This mostly to test future Mesa releases to
determine if bugs have been fixed. Used the new feature to
see if some long standing Mesa bugs have been fixed in newer
Mesa releases for the OS Mesa renderer.

* NPOT texture support used by vtkOpenGLContextDevice2D and
vtkOpenGL2ContextDevice2D fixed in Mesa >= 8

* reset glReadBuffer to quiet incomplete FBO error report in
vtkOpenGLVolumeGPURayCastMapper

* vtkOpenGLVolumeGPURayCastMapper tests pass in Mesa >= 9
except TestGPURayCastCompositeShadeMask. Explicitly disabled
the test for OS Mesa.

* disable FBO for Mesa < 8 because of TestBlurAndSobelPasses
TestSobelGradientMagnitudePass test failures

* disable vtkOpenGLHAVSVolumeMapper for OS Mesa without
Gallium llvmpipe renderer

* vtkCompositeRGBAPass needs GL_...