d3d12: Use Y channel on all stencil and store corrected swizzles separately

In DirectX the return value of stencil textures is actually located in the Y channel and we can not rely on the original swizzle holding the correct value in swizzle_g. In addition, mesa/st expects the sampler swizzles to correspond to the texture swizzles, therefore we can't override the swizzles in place and need to pass the values separately to the int texture lowering pass.

Replaces !256 (closed)