Emulate GL_PACK_SKIP_PIXELS and GL_PACK_SKIP_ROWS on macOS, where it
appears the OpenGL driver ignores these parameters.

Add WebGL 2.0-specific validation constraints for pixel pack and
unpack parameters.

Bug: angleproject:4849
Change-Id: Iab566299223e05484a009817acb1ed2816023823
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2303905
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>