rect_union_add takes a pixman_box32_t by value, and passes it along by
value to internal helpers. render_pass_mark_box_updated which is the
only caller receives the pixman_box32_t by reference, so just plumb it
through that way.
Results in a 13% improvement in CPU time when using the Vulkan renderer
on the stacked/clip200/1024 benchmarks on my machine.
Signed-off-by: Kenny Levinsen <kl@kl.wtf>