Allow scan shader to run only on warp

[pbbastian]

#7 needs to do a scan of counts, but the counts will always be of size 16, thus there is no need to sum up between thread groups. Global sync is needed however, so it might be best to do in another dispatch rather than just stuffing in at the end of the count shader.

A different shader program using a variant of the same compute shader is probably the way to go.