Only copy dirty depth buffers between FBOs

This is a bit of a hack.

We can remove this if/when we track them separately.  This may break a game that depends on the depth carrying over between several FBOs, but that's not extremely likely.

This improves performance in Gods Eater Burst.

I've only really tested it with Jeanne d'Arc, I don't know if this hurts other games that were improved by the depth copying.

-[Unknown]