vbo: Fix up in-place splitting for non-contiguous/indexed primitives.

vbo: Fix up in-place splitting for non-contiguous/indexed primitives.

The in-place splitting code wasn't dealing with index buffers at all
(and it was being called from vbo_split_prims for too big index
buffers, causing some occasional corruption).

Additionally, it wasn't taking into account primitives arrays with
non-contiguous indices (e.g. given prim[0].start = 0 and prim[1].start
= max_verts, it would happily call back the driver with (max_index -
min_index) still greater than max_verts, causing infinite recursion).

It still doesn't handle too large indexed vertex buffers: use
vbo_split_copy for that.