We just set the render area to the union of the scissor rects used in a pass.
Might help some games on some mobile hardware, a little bit.
Possibly #13464?
According to sage advice from themaister, this is just unnecessary overengineering.
Having them separate reduces the size of the renderpass key - we might be able to just
change that to an array lookup again. Will also make it easier to in the future add
depth-less render passes.
