draw: generate code for op == Over.

This change just does the mechanical codegen for the op == Over cases.
The actual compositing operator is still effectively Src. Fixing that,
which is less mechanical, will be a follow-up change.

Change-Id: I87805114d49e7ce7087066187a2f4c722a883c01
Reviewed-on: https://go-review.googlesource.com/8524
Reviewed-by: Rob Pike <r@golang.org>
2 files changed