Tweak image greyscale conversion

Instead of assuming everything that isn't exactly black to be white,
convert the image to greyscale and split at the middle of grey.
Anything darker than 50% grey is read as Black, while anything lighter
is read as White.

printer.go

@@ -446,7 +446,7 @@ func (p Printer) PrintImage8(img image.Image, density Density) error {
 					continue
 				}
 				c := color.GrayModel.Convert(img.At(x, y+i)).(color.Gray)
-				if c.Y == 0 {
+				if c.Y < 0x80 {
 					col |= 1
 				}
 			}
@@ -510,7 +510,7 @@ func (p Printer) PrintImage24(img image.Image, density Density) error {
 					}
 
 					c := color.GrayModel.Convert(img.At(x, (y+z*8)+i)).(color.Gray)
-					if c.Y == 0 {
+					if c.Y < 0x80 {
 						col |= 1
 					}
 				}