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 <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Nigel Tao (INACTIVE; USE @golang.org INSTEAD) <[email protected]>

Author: nigeltao

src/image/png/reader.go

@@ -51,6 +51,10 @@ func cbPaletted(cb int) bool {
 	return cbP1 <= cb && cb <= cbP8
 }
 
+func cbTrueColor(cb int) bool {
+	return cb == cbTC8 || cb == cbTC16
+}
+
 // Filter type, as per the PNG spec.
 const (
 	ftNone    = 0
@@ -898,6 +902,10 @@ func (d *decoder) parseChunk(configOnly bool) error {
 			if d.stage != dsSeenPLTE {
 				return chunkOrderError
 			}
+		} else if cbTrueColor(d.cb) {
+			if d.stage != dsSeenIHDR && d.stage != dsSeenPLTE {
+				return chunkOrderError
+			}
 		} else if d.stage != dsSeenIHDR {
 			return chunkOrderError
 		}