image/draw: optimize out some bounds checks.
We could undoubtedly squeeze even more out of these loops, and in the
long term, a better compiler would be smarter with bounds checks, but in
the short term, this small change is an easy win.
benchmark old ns/op new ns/op delta
BenchmarkFillOver-8
1619470 1323192 -18.29%
BenchmarkCopyOver-8
1129369 1062787 -5.90%
BenchmarkGlyphOver-8 420070 378608 -9.87%
On github.com/golang/freetype/truetype's BenchmarkDrawString:
benchmark old ns/op new ns/op delta
BenchmarkDrawString-8
9561435 8807019 -7.89%
Change-Id: Ib1c6271ac18bced85e0fb5ebf250dd57d7747e75
Reviewed-on: https://go-review.googlesource.com/14093
Reviewed-by: Rob Pike <r@golang.org>