Add support for printing on red/black tapes

My QL-800 refuses to print black only on a red/black tape.

I really don't like the added `rb` function parameter.
It would be best to be able to recognize such tapes, however
nothing of the sort is documented in official Brother documentation.

makeBitmapData{,RB} also look like they could be simplified further.

cmd/sklad/main.go

@@ -309,7 +309,7 @@ func printLabel(id string) error {
 	}
 
 	return printer.Print(&imgutil.LeftRotate{Image: label.GenLabelForHeight(
-		labelFont, id, mediaInfo.PrintAreaPins, db.BDFScale)})
+		labelFont, id, mediaInfo.PrintAreaPins, db.BDFScale)}, false)
 }
 
 func handleLabel(w http.ResponseWriter, r *http.Request) {