image/draw: add RGBA64Image fast path for RGBA dst
This should have been part of https://golang.org/cl/340049 but I
overlooked it. That commit added fast path code when the destination
image was *not* an *image.RGBA. This commit edits func drawRGBA.
name old time/op new time/op delta
RGBA1-4 5.11ms ± 1% 1.12ms ± 1% -78.01% (p=0.008 n=5+5)
RGBA2-4 8.69ms ± 1% 2.98ms ± 1% -65.77% (p=0.008 n=5+5)
Updates #44808.
Updates #46395.
Change-Id: I899d46d985634fc81ea47ff4f0d436630e8a961c
Reviewed-on: https://go-review.googlesource.com/c/go/+/351852
Trust: Nigel Tao <nigeltao@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>