image/draw: optimize paletted dst + uniform src

name            old time/op  new time/op  delta
PalettedFill-4  5.74ms ± 1%  0.01ms ± 1%  -99.78%  (p=0.008 n=5+5)
PalettedRGBA-4  3.34ms ± 3%  3.33ms ± 0%     ~     (p=0.690 n=5+5)

Fixes #35938

Thanks to pjbgtnj for the suggestion.

Change-Id: I07b494482cce918f556e196c5a4b481b4c16de3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/230118


Run-TryBot: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>