The new RGBA64At method is equivalent to the existing At method (and the
new SetRGBA64 method is equivalent to the existing Set method in the
image/draw package), but they can avoid allocations from converting
concrete color types to the color.Color interface type.
Also update api/go1.17.txt and doc/go1.17.html
Fixes #44808
Change-Id: I8671f3144512b1200fa373840ed6729a5d61bc35
Reviewed-on: https://go-review.googlesource.com/c/go/+/311129
Trust: Nigel Tao <nigeltao@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Rob Pike <r@golang.org>