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
1 file changed