Tweak PrintImage24

Removed the double loops in PrintImage24.  Instead of converting the
entire image before sending data to the printer, send each row as soon
as it has been converted.  PrintImage8 already did this.

Author: zorchenhimer

printer.go

@@ -492,14 +492,13 @@ func (p Printer) PrintImage24(img image.Image, density Density) error {
 		return fmt.Errorf(errMsg, err)
 	}
 
-	imgBytes := [][]byte{}
+	command := []byte{ESC, 0x2A, byte(density + 32), byte(imgRect.Max.X), byte(imgRect.Max.X >> 8)}
 
-	// First convert the image data to the 1-bit data format.
 	// 24 dot density (meta row is 24 dots tall (3 bytes))
 	for y := 0; y < imgRect.Max.Y; y += 24 {
-		metaRow := []byte{}
-		for x := 0; x < imgRect.Max.X; x++ {
+		row := []byte{}
 
+		for x := 0; x < imgRect.Max.X; x++ {
 			for z := 0; z < 3; z++ {
 				col := byte(0)
 				for i := 0; i < 8; i++ {
@@ -514,16 +513,10 @@ func (p Printer) PrintImage24(img image.Image, density Density) error {
 						col |= 1
 					}
 				}
-				metaRow = append(metaRow, col)
+				row = append(row, col)
 			}
-
 		}
-		imgBytes = append(imgBytes, metaRow)
-	}
 
-	// Next send the data to the printer.
-	command := []byte{ESC, 0x2A, byte(density + 32), byte(imgRect.Max.X), byte(imgRect.Max.X >> 8)}
-	for _, row := range imgBytes {
 		err = p.SetLineSpacing(0)
 		if err != nil {
 			return fmt.Errorf(errMsg, err)