glRenderbufferStorageMultisampleCoverageNV used on Mesa, despite not having NV_framebuffer_multisample_coverage

[AnyOldName3]

glRenderbufferStorageMultisampleCoverageNV is provided by the NV_framebuffer_multisample_coverage extension for implementing what Nvidia call Coverage Sampled Antialiasing. It's used in OSG by osg::RenderBuffer::getObjectId following a check to osg::GLExtensions::isRenderbufferMultisampleCoverageSupported() in favour of the non-vendor-specific glRenderbufferStorageMultisample.

osg::GLExtensions::isRenderbufferMultisampleCoverageSupported() just checks whether or not the function pointer is null, which fails on Mesa as it exports the function, despite not implementing the relevant extension, and OSG doesn't check before setting the function pointer here. It seems that on Mesa, the call is a no-op, as I get incomplete FBO errors when OSG uses that path, but not if I replace the call with the non-coverage variant, and I can't find an actual implementation in the Mesa source, but can for the regular variant.

I expect that the most sensible solution would be to check for the presence of the actual extension, either before setting the function pointer or in isRenderbufferMultisampleCoverageSupported.

[glebm]

@robertosfield I can work on a PR for this if you have a preferred solution in mind

Should we guard the assignment with an if (and similar for glRenderbufferStorageMultisample):

if (isGLExtensionSupported(contextID, "NV_framebuffer_multisample_coverage"))
  setGLExtensionFuncPtr(glRenderbufferStorageMultisampleCoverageNV, "glRenderbufferStorageMultisampleCoverageNV", validContext);

Or should this be another two boolean fields in GLExtensions?