HLSL: Support depth comparison texture sampling in SM 2/3.

[rdb]

In DX9, all samples of a depth texture result in a depth comparison (in fact, disabling this requires use of the special INTZ format, only available on DX10+ hardware). This works with all 4-component texture sample functions (tex2Dlod, tex2Dbias, tex2Dproj).

This PR provides access to this functionality by enabling use of the Dref texture sample instruction and passing in the depth compare value into the appropriate register. It assumes that there is indeed a depth texture bound to the appropriate register (which is the application's responsibility, and not something we can check).

Even though the regular texld instruction does support this as well, HLSL does not (to my knowledge) allow access to this variant since tex2D only accepts 2 coordinates. To work around this, this PR turns an OpImageSampleDrefImplicitLod into a tex2Dproj with a w coordinate of 1.0, which is effectively equivalent.

Thanks for your consideration!

[HansKristian-Work]

I'd like to see a test here to see what is going on.

[rdb]

I added a test for all the samplers supported by SM 3.0.

As you can see it's just a matter of passing the dref into the third coordinate. I verified that the resulting shader works correctly for shadow mapping on both Intel and NVIDIA hardware, and furthermore, this is also what the Cg compiler is producing when using a shadow sampler and targeting SM 3.0 (aside from the fact that it can directly generate a texld instead of texldp, but that has no effect on the result).

Thanks!

[HansKristian-Work]

Also missing reference/opt/shaders-hlsl/frag/tex-sampling.sm30.frag to be committed.

[rdb]

I just went ahead and just changed the second coordinate to 0.0, because it's clearer. I verified that the resulting shader still works correctly with a 1D depth map.

I also added a return swizzle and added the optimized reference file as requested.

[HansKristian-Work]

LGTM, thanks.