image/draw: add RGBA64Image fast path

image/draw: add RGBA64Image fast path

name               old time/op  new time/op  delta
GenericOver-4      15.0ms ± 1%   2.9ms ± 1%  -80.56%  (p=0.008 n=5+5)
GenericMaskOver-4  7.82ms ± 4%  1.69ms ± 2%  -78.38%  (p=0.008 n=5+5)
GenericSrc-4       6.13ms ± 3%  1.66ms ± 1%  -72.90%  (p=0.008 n=5+5)
GenericMaskSrc-4   11.5ms ± 1%   2.0ms ± 0%  -82.77%  (p=0.008 n=5+5)

Updates #44808.

Change-Id: I131cf6fad01708540390a8012d8f2a21e849fe9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/340049
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Nigel Tao <nigeltao@golang.org>