draw: distinguish YCbCr fast paths by their chroma subsample ratios.

These code paths aren't actually fast yet. That will be a follow-up
change.

Change-Id: I814992573cc6af422e49d0ddf336003e662897a5
Reviewed-on: https://go-review.googlesource.com/7791
Reviewed-by: Rob Pike <r@golang.org>
2 files changed