Reland "Start on custom mesh API"

This is a reland of e1e1c03229455a9e5b20eefa0ebbed645a216f51

Original change's description:
> Start on custom mesh API
>
> Bug: skia:12720
> Change-Id: I8ed0555f1c5b8b0e97e73a657635dac2c0df0ccf
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/482457
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Brian Salomon <bsalomon@google.com>

Bug: skia:12720
Change-Id: I5c54f37949d8bd45ea606264c439912b65158b79
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/487220
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
37 files changed