Integrate vtxarray with its related functions

[no-lex]

The vertex array object vtxarray is currently a POD struct which contains no member functions, despite the presence of free functions which are in practice essentially member functions of that object. Merging vtxarray member functions with vtxarray will make it more clear under which scope functions involving vtxarray belong, and reduce the number of free functions floating around in the global scope.

Eventually this should allow making much of or all of vtxarray's data private, making its interface much more obvious to other developers.