fix palette conversions having empty holes if the color was not encountered

src/Program.cs

@@ -212,6 +212,13 @@ namespace Palettizer
 					else
 					{
                         palette.AddColor(color);
+
+						var grayscaleColor = palette.ColorToGrayscaleMap[color];
+
+						grayscaleImage[i] = grayscaleColor;
+						grayscaleImage[i + 1] = grayscaleColor;
+						grayscaleImage[i + 2] = grayscaleColor;
+						grayscaleImage[i + 3] = color.A;
 					}
 				}
 			}