gallium/u_vbuf: handle indirect multidraws correctly and efficiently (v3)

v2: need to do MAX{start+count} instead of MAX{count}
    added piglit tests
v3: use malloc

Cc: 18.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 0f79b201)