WebGPU: Implement viewport and scissor Tested using samples. dEQP tests covering viewport and scissor require more shader translation to be completed. Bug: angleproject:0 Change-Id: Ia76f2edeea5a07af1881704a21fafb64ab7df43a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5756127 Reviewed-by: Liza Burakova <liza@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>