CopyTextureCHROMIUM: Validate the texture target enums first. INVALID_ENUM should be generated before any INVALID_VALUE errors based on the texture state. BUG=angleproject:1932 Change-Id: If74440602d56b1a86fa24ff87c38d2b0dec2bfb2 Reviewed-on: https://chromium-review.googlesource.com/744448 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>