Use 4 colors in Checkered test shader This shader previously output red and green in a checkered pattern (with four regions). This makes it hard to verify that the output is not rotated / flipped. This change makes the shader output: +--------+--------+ | red | blue | +--------+--------+ | green | yellow | +--------+--------+ Bug: angleproject:4901 Change-Id: I02723306dec17d1419f11d9f76e96b1cefc3bf06 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2498261 Reviewed-by: Ian Elliott <ianelliott@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>