Types for offsets: usize vs ?usize vs ?*c_void

[silversquirl]

The OpenGL API has a habit of using void pointers for offsets into buffer objects, eg. in glVertexAttribPointer, glDrawElements, etc.
The only reason I see for this being done is so pointer arithmetic can be used to remove the need to multiply things by sizeof (T), however Zig does not have pointer arithmetic, leaving this as purely confusing and difficult to handle, without any ergonomic benefit.

zgl currently uses ?usize for the offset parameter to vertexAttribPointer, and ?*const c_void for the parameter to glDrawElements and glDrawElementsInstanced. The way the former is converted into the C type will cause checked undefined behaviour if 0 is passed in, and the latter is difficult to deal with as discussed above.

I propose changing all of these functions, as well as any future functions making use of this kind of offset, to use usize, which can be converted into a *allowzero c_void for passing to C.