Refactor CopyTexImage validation. Move the common validation code to a shared base function. BUG=angle:571 Change-Id: Id70b413b408a21f0a8933cfd4a09e261e637bae1 Reviewed-on: https://chromium-review.googlesource.com/200559 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>