vbo/dlist: implement primitive merging

vbo/dlist: implement primitive merging

Merge consecutive primitives using the same mode while constructing the index buffer.

This improves performance a lot (x3 - x10) SPECviewperf13 snx-03 test by reducing the
number of draw calls per frame.
Here are some numbers for 4 of the tests:

      | Num draw calls |     GPU-load    |
------|----------------|-----------------|
      | Before | After |  Before | After |
------|--------|-------|---------|-------|
test1 |  390k  |  16k  |   68%   |  90%  |
test2 |  370k  |  16k  |   40%   |  90%  |
test3 |  1.2M  |  35k  |   38%   |  78%  |
test10|  3.5M  |  35k  |   36%   |  58%  |

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7078>