add indicator in gl.xml that particular pointers also (or only) accept offsets

Various functions (`glVertexAttribPointer`, etc) have arguments that accept `void *` or similar but also accept (or in core, only accept) an offset into a bound buffer. It would be nice to indicate that in the XML.

(I have a partial list, so could submit a PR for those, just need to know how to mark it in the XML. `group="OffsetOrPointer"` maybe, or new attribute?)