image: add Uniform.RGBA64At and Rectangle.RGBA64At

These types already implemented the Image interface. They should also
implement the RGBA64Image interface (new in Go 1.17)

Updates #44808

Change-Id: I9a2b13e305997088ae874efb95ad9e1648f94812
Reviewed-on: https://go-review.googlesource.com/c/go/+/331570


Trust: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>