ES31: Fix the issue when rendering against compute program. It is a undefined behavior in gles spec, but we should generate an error. This change also refactored the coding style for shader in the test. BUG=angleproject:2260 Change-Id: I7b480e8b66486d9954f7c7f6e8683298e94b6ad7 Reviewed-on: https://chromium-review.googlesource.com/764797 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>