image/png: allow tRNS chunk without a PLTE chunk.

While https://www.w3.org/TR/PNG/#5ChunkOrdering says that tRNS's
ordering constraint is "After PLTE; before IDAT", it is legal for a tRNS
chunk to occur without a PLTE chunk at all, for greyscale and truecolor
transparency as opposed to palette-based transparency. See
https://www.w3.org/TR/PNG/#11transinfo

Fixes #17511.

Change-Id: I047b0b01d78a1cda65e00eeac229bb972cda431d
Reviewed-on: https://go-review.googlesource.com/32139Reviewed-by: Rob Pike <r@golang.org>

src/image/png/reader.go

@@ -709,9 +709,13 @@ func (d *decoder) parseChunk() error {
 		d.stage = dsSeenPLTE
 		return d.parsePLTE(length)
 	case "tRNS":
+		if cbPaletted(d.cb) {
 			if d.stage != dsSeenPLTE {
 				return chunkOrderError
 			}
+		} else if d.stage != dsSeenIHDR {
+			return chunkOrderError
+		}
 		d.stage = dsSeentRNS
 		return d.parsetRNS(length)
 	case "IDAT":