draw: RGBA src fast path for scaling.

benchmark                     old ns/op      new ns/op      delta
BenchmarkScaleSrcRGBA         15124800       2091946        -86.17%

Change-Id: Id8d3088793ebf1d75b929fcf6945987817e87463
Reviewed-on: https://go-review.googlesource.com/6234
Reviewed-by: Rob Pike <r@golang.org>
2 files changed