Draft: tests: Make ddxddy.shader_test pass for different GPUs.

On my Nvidia GeForce GTX 1050 Ti this test is not passing because of considerably different numeric results.

As Giovanni pointed out, this is because my GPU uses the fine derivate and not the coarse derivate to implement ddx() and ddy().

Testing both ddx_coarse()|ddy_coarse() and ddx_fine()|ddy_fine() on the WARP driver shows that both these derivates are the same in coordinates where both X and Y are even, i.e. the first pixel of each 2x2 quad. So the test was modified to only probe on these coordinates.

The new expected values were obtained from running the test using the WARP driver, and ulps adjusted for my GPU. However, this MR is marked as a draft because I would like to know if the test passes on other GPUs.