OP_DIFF and OP_LUMA roll over and under test cases

I wasn't explicitly testing the wrapping behavior of the diff and luma
op codes.

src/main.rs

@@ -336,6 +336,44 @@ mod test {
         assert_eq!(result, expected);
     }
 
+    #[test]
+    fn decoder_unpack_diff_rollover() {
+        let init_pixel = PixelRGBA::new(255, 255, 255, 255);
+        let compressed = [
+            (QOI_OP_DIFF | 0b0011_1111), // +1s
+            (QOI_OP_DIFF | 0b0011_1111), // +1s
+            (QOI_OP_DIFF | 0b0010_1010), // +0s, could have been an index or a run, probably
+        ];
+        let expected = [
+            PixelRGBA::new(0, 0, 0, 255), // 255 rollover to 0
+            PixelRGBA::new(1, 1, 1, 255), // +1s
+            PixelRGBA::new(1, 1, 1, 255), // holds at 1s
+        ];
+
+        let decoder = Decoder::new_with_previous_pixel(compressed.into_iter(), init_pixel);
+        let result: Vec<PixelRGBA> = decoder.collect();
+        assert_eq!(result, expected);
+    }
+
+    #[test]
+    fn decoder_unpack_diff_rollunder() {
+        let init_pixel = PixelRGBA::new(0, 0, 0, 255);
+        let compressed = [
+            (QOI_OP_DIFF | 0b0001_0101), // -1s
+            (QOI_OP_DIFF | 0b0001_0101), // -1s
+            (QOI_OP_DIFF | 0b0010_1010), // 0s
+        ];
+        let expected = [
+            PixelRGBA::new(255, 255, 255, 255),
+            PixelRGBA::new(254, 254, 254, 255),
+            PixelRGBA::new(254, 254, 254, 255),
+        ];
+
+        let decoder = Decoder::new_with_previous_pixel(compressed.into_iter(), init_pixel);
+        let result: Vec<PixelRGBA> = decoder.collect();
+        assert_eq!(result, expected);
+    }
+
     #[test]
     fn decoder_unpack_luma() {
         // red and blue diffs are relative to the green channel as (dr - dg) and (db - dg)
@@ -364,6 +402,32 @@ mod test {
         assert_eq!(result, expected);
     }
 
+    #[test]
+    fn decoder_unpack_luma_rollover() {
+        let init_pixel = PixelRGBA::new(255, 255, 255, 255);
+        let compressed = [
+            (QOI_OP_LUMA | 0b0011_1111), (0b1111_1111), // Diff (31, 7, 7) -> Pix (38, 31, 38)
+        ];
+        let expected = PixelRGBA::new(37, 30, 37, 255);
+
+        let mut decoder = Decoder::new_with_previous_pixel(compressed.into_iter(), init_pixel);
+        let result = decoder.next().expect("Oops, didn't get a Pixel back from the Decoder");
+        assert_eq!(result, expected);
+    }
+
+    #[test]
+    fn decoder_unpack_luma_rollunder() {
+        let init_pixel = PixelRGBA::new(0, 0, 0, 255);
+        let compressed = [
+            (QOI_OP_LUMA | 0b0001_0011), (0b1100_0011) // Diff(-13, 4, -5) -> Pix (-9, -13, -18)
+        ];
+        let expected = PixelRGBA::new(247, 243, 238, 255);
+        
+        let mut decoder = Decoder::new_with_previous_pixel(compressed.into_iter(), init_pixel);
+        let result = decoder.next().expect("Oops, didn't get a Pixel back from the Decoder");
+        assert_eq!(result, expected);
+    }
+
     #[test]
     fn decoder_unpack_run() {
         let compressed = [