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

[3b]

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?)

[oddhack]

I think group= would be OK. There's already a group="String" used in a few places and that's not a scalar type (nor is there any attempt to define it in the XML).

[3b]

hmm, looks like a few already have a group, like "CompressedTextureARB" in

  <command>
            <proto>void <name>glCompressedTexImage1D</name></proto>
<!-- ... -->
            <param group="CompressedTextureARB" len="imageSize">const void *<name>data</name></param>
<!-- ... -->
        </command>

not sure how to handle that.