stop searching for color matches if this color is an exact match

probably very rarely ever an optimization in real-world use, but it
still seems like the logical thing to do

libretrogd/src/graphics/palette.rs

@@ -483,6 +483,9 @@ impl Palette {
         for (index, color) in self.colors.iter().enumerate() {
             let (this_r, this_g, this_b) = from_rgb32(*color);
 
+            if r == this_r && g == this_g && b == this_b {
+                return index as u8;
+            } else {
                 // this comparison method is using the sRGB Euclidean formula described here:
                 // https://en.wikipedia.org/wiki/Color_difference
 
@@ -495,6 +498,7 @@ impl Palette {
                     closest_distance = distance;
                 }
             }
+        }
 
         closest
     }