fixed vec3 size differences and mod calculation of skslc msl backend

Bug: skia:
Change-Id: I5dc6d956463845627a1a2debee5ac8856e5e2e8d
Reviewed-on: https://skia-review.googlesource.com/132500
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
2 files changed