Incorrect roughness to mip level mapping in PBR shaders

When using IBL with PBR shaders the calculation of the mip level from the roughness value relies upon the env maps having a size on 2k (I think). When using smaller env maps, we end up calculating mip levels that are too high and contain bad/missing data leading to broken lighting for very rough materials.

Check this roughness to mip mapping function and try to use one independent of the env map size or expose properties to control it.