fixed interface/in/out struct for mvk integration and added a few missing builtins to skslc msl backend

Bug: skia:
Change-Id: Id514dc6af64bb2a5eb71f2161ceba549a8a6836d
Reviewed-on: https://skia-review.googlesource.com/131881
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
2 files changed