Fix sample qualifier validation * `smooth sample` is the same as `sample` * `flat sample` is the same as `flat` Bug: angleproject:3589 Change-Id: I8615fa73f4b0cea729b8e09d86f1b8183669de2b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4356666 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>