D3D12: NULL RTVs
When we get a NULL-surface passed to pipe_context::set_framebuffer_state()
, we currently derference them. That leads to a crash.
So this MR changes this to allocate two RTVs, one for msaa and without, and use those instead.
Unlike the NULL SRVs, we pick R8G8B8A8_UNORM
, to try to avoid unexpected problems with stuff like logic-op etc.