Be much more strict about extension support

Be much more strict about extension support

OpenGL ES has added bits and pieces of full OpenGL functionality, an the
extension generator was thinking an extension was supported when really
only a few tokens from it were.  Now all tokens must be either supported
or explicitly marked as optional for the extension to be made available.

Some features may be temporarily unavailable on OpenGL ES builds as I go
over the extensions and decide how to fix them.