Cube texture sampling functions which take explicit gradients require
the gradients to divided by the same factor used to project the texture
coordinates onto the cube. Previously we assumed they were already in
the projected coordinate space.

Change-Id: I2825df17bee74b551b8d7491c050b7766653a65c
Reviewed-on: https://swiftshader-review.googlesource.com/13552Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>