WebGPU: Implement no-op gl_PointSize Support writing to the gl_PointSize builtin but do not pass it to the next shader stages. Bug: angleproject:392604862 Change-Id: I845a3edeb0ce8e7ea41a5e96e5f3b443100f2cbf Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6194555 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Matthew Denton <mpdenton@chromium.org>