draw: add mask fields to Options.

This change only adds the fields, more or less. Follow-up changes will
actually honor the masks.

Change-Id: I81411dc1aac4b3c846dcdf13e2cb0b5cd60fb2b4
Reviewed-on: https://go-review.googlesource.com/8902
Reviewed-by: Rob Pike <r@golang.org>
3 files changed