glapi: sort ARB extensions by number

glapi: sort ARB extensions by number

And add comments to fill in for extensions that aren't there.

Noticed the comment about "ARB extensions sorted by extension number"
didn't extend to the <xi:include> directives when it became clear
GL_ARB_texture_rg was missing, going by the error message seen here:

https://bugs.freedesktop.org/show_bug.cgi?id=44039

This makes it easier to notice in the future if an extension is missing
when it shouldn't be.

Reviewed-by: Brian Paul <brianp@vmware.com>