Vulkan: Remove ImageHelper::setFirstAllocatedLevel and Add assertion ImageHelper::mFirstAllocatedLevel is set via ImageHelper::initExternal. There isn't a good use to set the mFirstAllocatedLevel when mImage is invalid, and semantically it is wrong to try to get mFirstAllocatedLevel or format while its storage is invalid (i.e., not allocated). This CL removes ImageHelper::setFirstAllocatedLevel() API all together and added assertion that image is valid when ImageHelper::getFirstAllocatedLevel and getActualFormat and various other get APIs whose value are set during initExternal call. Bug: angleproject:6532 Change-Id: I9c97f488b3e959443fec632540e7f0906a49dc13 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3209838 Commit-Queue: Charlie Lao <cclao@google.com> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>