support cull useless pass in render graph

[yangfengzzz]

After reading render graph code, I found the usage of render graph only is to manage transcient resource and set barrier. I think rg should use to cull useless pass too, if the pass is note releated to the final image(backboard resource)

or maybe there already have this feature in Kajiya? I don't find any related code.

[h3r2tic]

Hey! Yeah, render graphs can certainly remove unnecessary passes, but as you correctly note, the implementation in kajiya doesn't.

What's the purpose of this issue? Is this something that you'd like to contribute, or just wanted to ask a question?