u_vbuf: implement another upload codepath which unrolls indices

u_vbuf: implement another upload codepath which unrolls indices

Improves performance from cca 1 fps to 23 fps in Cogs.
This new codepath is not always used, instead, there is a heuristic which
determines whether to use it. Using translate for uploads is generally
slower than what we have had already, it's a win only in a few cases.