WIP: D3D12: support rendering to cube arrays
I'm far from sure that this is the right thing to do, as I'm having a hard time finding the right docs on the D3D12 side.
It makes the "spec@arb_framebuffer_object@fbo-incomplete" piglit test go from crash to pass. No other test seems consistently affected by this, which is a bit strange to me.
WIP because this is an OpenGL 4.0 feature.