BUG: Fixed and simplified vtkOpenGLExtensionManager and updated OpenGL header files (Merge Francois's changes from main tree into VTK-5-0 branch...) The following file revisions were copied directly into the VTK-5-0 branch for this commit:
Rendering/vtkOpenGLExtensionManager.cxx            1.27
Rendering/vtkOpenGLExtensionManager.h              1.9
Rendering/Testing/Cxx/LoadOpenGLExtension.cxx      1.18
Utilities/ParseOGLExt/ParseOGLExt.cxx              1.22
Utilities/ParseOGLExt/headers/glext.h              1.3
Utilities/ParseOGLExt/headers/glxext.h             1.4
Utilities/ParseOGLExt/headers/wglext.h             1.2
VolumeRendering/vtkOpenGLVolumeTextureMapper3D.cxx 1.11