draw: refactor codegen to use argf instead of fmt.Sprintf.
draw: refactor codegen to use argf instead of fmt.Sprintf.

Change-Id: Iee4cb1b605c381b75a688a5e65e9afc4d34df193
Reviewed-on: https://go-review.googlesource.com/8900
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

golang/image