fixed the size in bytes of vec3s to match Metal standard in Metal gpu backend

Bug: skia:
Change-Id: Iba46b7eff46cfd05c3b65ba2d38f10dce16e9481
Reviewed-on: https://skia-review.googlesource.com/146764
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
3 files changed