d3d12: cubemap-array support
We've already done most of the work to get cubemap-arrays working on D3D12. This finishes up the last bits, and enables it.
For some strange reason, the
spec/arb_texture_cube_map_array/arb_texture_cube_map_array-cubemap piglit-test fails on CI. It passes locally for me, both on Intel and on WARP. I wonder if this could be due to an issue on older versions of WARP.