image/draw: fix double-draw when the dst is paletted.

image/draw: fix double-draw when the dst is paletted.

The second (fallback) draw is a no-op, but it's a non-trivial amount of work.

Fixes #11550.

benchmark               old ns/op     new ns/op     delta
BenchmarkPaletted-4     16301219      7309568       -55.16%

Change-Id: Ic88c537b2b0c710cf517888f3dd15cb702dd142f
Reviewed-on: https://go-review.googlesource.com/11858
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>