mesh.c: Fix bug with texcoord data stride
mesh: Add buffer fill API with modified submesh_metadata

submesh_metadata now contains base_vertex and base_triangle. These
base array indices need to be set when setting up various submeshes.
The first submesh will have base_vertex and base_triangle set to zero.
All submeshes after that will have their base vertex/triangle set to
the base vertex/triangle of the previous submesh plus that previous
submesh's stride.

This was tested in gDEBugger and doesn't appear to have any issues
so far, but there have yet to be any actually graphics tests using
the mesh API that can be confirmed as working by a warm body.