The LOD value was not being computed for 3D texture sampling when no
mipmapping is done. We still need the LOD in case the minification and
magnification filters differ.

Change-Id: If32d7aaf8a92ace33bcc6ca6f0b60fe33c7e771b
Reviewed-on: https://swiftshader-review.googlesource.com/15588Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>