Avoid accessing pixel depth directly in tcuTexCompareVerifier.
Avoid accessing pixel depth directly in tcuTexCompareVerifier.

- Avoid accessing depth directly to make structure idential to
  tcuTexLookupVerifier and to make refactoring easier.
- Add sanity checks to isGatherOffsetsCompareResultValid().

Change-Id: I3afc3015edc775c1baf13eea94a781bc6f6e4829

KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,chadversary/deqp,geekboxzone/mmallow_external_deqp,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,chadversary/deqp,geekboxzone/mmallow_external_deqp,chadversary/deqp,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/VK-GL-CTS,chadversary/deqp,chadversary/deqp,KhronosGroup/Vulkan-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/Vulkan-CTS,chadversary/deqp,geekboxzone/mmallow_external_deqp,geekboxzone/mmallow_external_deqp