We used to compute the cube map LOD by projecting all sampling rays onto the
same face (arbitrarily chosen). This changes it to intersect the rays with the
cube and use the 3D distance between them.

Change-Id: If90b29cb4c13e6128ce6642eb28bb92602e783df
Reviewed-on: https://swiftshader-review.googlesource.com/5160Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>