Clear color mask to implement missing PS
Vulkan does support rendering without PS, though it leaves undefined color values, so mask them out with the colorWriteMask.
I tried running tests, but test_draw_depth_no_ps passes even without zeroing colorWriteMask on both a B580 and an RX 6600 XT, so I don't know that I'm testing the GPU that motivated inserting the dummy PS in the first place. No regressions on the two GPUs I tested though.