Fix derivate offset in cube map verification code
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

maurossi/deqp,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,maurossi/deqp,chadversary/deqp,maurossi/deqp,android-ia/platform_external_deqp,KhronosGroup/Vulkan-CTS,android-ia/platform_external_deqp,MIPS/external-deqp,geekboxzone/lollipop_external_deqp,MIPS/external-deqp,geekboxzone/lollipop_external_deqp,geekboxzone/mmallow_external_deqp,geekboxzone/lollipop_external_deqp,MIPS/external-deqp,android-ia/platform_external_deqp,Omegaphora/external_deqp,maurossi/deqp,KhronosGroup/Vulkan-CTS,geekboxzone/lollipop_external_deqp,KhronosGroup/Vulkan-CTS,maurossi/deqp,android-ia/platform_external_deqp,MIPS/external-deqp,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,MIPS/external-deqp,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,android-ia/platform_external_deqp,Omegaphora/external_deqp,KhronosGroup/VK-GL-CTS,chadversary/deqp,chadversary/deqp,Omegaphora/external_deqp,Omegaphora/external_deqp,Omegaphora/external_deqp,geekboxzone/lollipop_external_deqp,geekboxzone/mmallow_external_deqp,geekboxzone/lollipop_external_deqp,MIPS/external-deqp,Omegaphora/external_deqp,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,geekboxzone/mmallow_external_deqp,geekboxzone/mmallow_external_deqp,chadversary/deqp,chadversary/deqp,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,chadversary/deqp,maurossi/deqp,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,android-ia/platform_external_deqp