Draft: dzn: Add support for VK_EXT_multi_draw
Based on !15912 (merged) (needs the draw_id sysval).
Multi draw is pretty similar to regular draws except it avoids re-setting the context on each draw. There's probably not much to be gained since we already track dirty states and only update things that changed when draws are issued, so I'm not entirely sure it's worth supporting this extension (hence the Draft: tag). Posting it for the record, since I had it implemented to increase dEQP-VK.draw.* test coverage.
Note that the triangle-fan index build process could be more efficient if we were creating a single index buffer containing entries for all draws.