Convert mask to 8-bit instead of only adjusting bit depth

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: ppouchin

src/main/java/fr/igred/nucleus/utils/Thresholding.java

@@ -132,7 +132,8 @@ public static void convertToMask(ImagePlus input, double lowerThreshold, double
 				}
 			}
 		}
-		stack.setBitDepth(8); // Ensure mask is 8-bit
+		ImageConverter converter = new ImageConverter(input);
+		converter.convertToGray8(); // Ensure mask is 8-bit
 	}
 
 }