draw: merge draw_go117.go into draw.go

This can be done as Go 1.21 and 1.20 are the supported
versions by now, and Go 1.16 is long since unsupported.

Change-Id: Ib90f796d6c38dbb9ffd99d56faf344ee9784f9bc
Reviewed-on: https://go-review.googlesource.com/c/image/+/526896
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nigel Tao (INACTIVE; USE @golang.org INSTEAD) <nigeltao@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

draw/draw.go

@@ -47,6 +47,12 @@ func (floydSteinberg) Draw(dst Image, r image.Rectangle, src image.Image, sp ima
 // Image is an image.Image with a Set method to change a single pixel.
 type Image = draw.Image
 
+// RGBA64Image extends both the Image and image.RGBA64Image interfaces with a
+// SetRGBA64 method to change a single pixel. SetRGBA64 is equivalent to
+// calling Set, but it can avoid allocations from converting concrete color
+// types to the color.Color interface type.
+type RGBA64Image = draw.RGBA64Image
+
 // Op is a Porter-Duff compositing operator.
 type Op = draw.Op