Fix array texture LOD computation.

2D array textures use 2D LOD calculations, not 3D.

Change-Id: I0ce834bc4afb63dfeba3d9662105ffdaa07028da
Reviewed-on: https://swiftshader-review.googlesource.com/4716
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>