Softpipe: Support explicit derivatives, rework LOD evaluation and correct Cube face selection
This MR reworks the LOD evaluation so that explicit derivatives can be supported. It also corrects the Cube face selection. All in all the series fixes about 4000 tests from deqp gles31 master set related to cube textures and another bunch from the gles3 master set related to texgrad test, as well as a number of piglits, all without any regressions.
The major incentive for especially for fixing the cube map face lookup is because we use softpipe as a backend in the virgl CI on fdo.
Thanks for reviewing.