Validate target parameter in CopyTexImage calls before using to check mip level Would cause UNREACHABLE to be triggered if target was invalid, and INVALID_VALUE to be returned instead of INVALID_ENUM if it were the only erroneous argument. Change-Id: I2606e77379fa2832a50b3a299e6474a4b2f68afa Reviewed-on: https://chromium-review.googlesource.com/189701 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Shannon Woods <shannonwoods@chromium.org>