Metal requires the vertex attribute offset to be multiples of 4, and its
stride must not be less than attribute's size.
This should fix the WebGL's test:
conformance/attribs/gl-vertexattribpointer-offsets.html

Bug: angleproject:4846
Bug: angleproject:2634
Change-Id: I0784a8ccaedd5e6c58a266243bfa94ba36e53e11
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2374829
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>