tests: Test coarse and fine derivates and demand less precision from ddx() and ddy().

On my Nvidia GeForce GTX 1050 Ti ddxddy.shader_test doesn't pass 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().

For this reason, the result for ddx|ddy() is quantized so that the test passes if the GPU uses either coarse or fine derivates.

Additionally, tests for both ddx_coarse|ddy_coarse() and ddx_fine|ddy_fine() are added, that expect a more precise result.