Fix derivate offset in cube map verification code

To accomodate implementations that use same derivative for a 2x2 pixel
quad when computing LOD, cube map verification code samples derivatives
in 3x3 neighborhood and computes bounds based on that.

This change fixes a typo that prevented the code from considering
(+1, +1) offset.

Bug: 19170879
Change-Id: I2de7aa422fc2fa3ff4432e802bb98c90aedeb548
1 file changed