dispinterfaces now have a type of RPC_FC_IP instead of 0, so this code
will never be reached.

I suspected that in this situation d3d writes to the z buffer, but the
test shows that I was mistaken

Add a test for this behaviour.

The render target should be left alone if the render target argument is
NULL. WineD3D calls should be done inside the critical section.

Turns out the original fix was correct for fixed function, but for the
wrong reason. The shader path was already correct. This fixes a
regresssion introduced by 932e95c1.