YUV should support on all the shader stages * The spec requires all shader stages should support YUV. Therefore, move YUV related functions out of the switch. * Add a new end2end test that sampling YUV in the vertex shader. Bug: angleproject:391919465 Change-Id: I7811d419d5f1461c2499c522a902c3b2149289a1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6263325 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>