image/png: allow both PLTE and tRNS chunks for TrueColor

image/png: allow both PLTE and tRNS chunks for TrueColor

Prior to this commit, png.Decode would allow TrueColor PNG images that
have one but not both of PLTE and tRNS chunks.

Fixes #54142

Change-Id: I259c1fff86a0aa5640dbadf7ad834e05fbd1430c
Reviewed-on: https://go-review.googlesource.com/c/go/+/424916
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Nigel Tao (INACTIVE; USE @golang.org INSTEAD) <nigeltao@google.com>