The old code got the accumulation a bit wrong. For one thing, it always
accumulates with whatever was there instead of resetting to empty each
time. For another, it sets with with y and height with x when it writes
back to the resource. This is also all too complicated because it
converts between pipe_box, u_rect, and VkRect2D on every iteration.
Instead, there are helpers in util/box.h which will do most of this work
for us and they're correct. Let's just use them to get rid of the bugs
and make everything simpler and more obvious at the same time.
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12194
Fixes: 3d38c9597f ("zink: hook up KHR_partial_update")
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33855>
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