Support 64-bit encoding

Note that this adds the LONG8 data type from BigTIFF because of bounds
requiring TiffValue. However, this datatype should not actually be
written to tags.

tests/encode_images.rs

@@ -238,6 +238,67 @@ fn test_rgb_u32_roundtrip() {
     }
 }
 
+#[test]
+fn test_gray_u64_roundtrip() {
+    let img_file =
+        File::open("./tests/images/gradient-1c-64b.tiff").expect("Cannot find test image!");
+    let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
+    assert_eq!(decoder.colortype().unwrap(), ColorType::Gray(64));
+
+    let image_data = match decoder.read_image().unwrap() {
+        DecodingResult::U64(res) => res,
+        _ => panic!("Wrong data type"),
+    };
+
+    let mut file = tempfile::tempfile().unwrap();
+    {
+        let mut tiff = TiffEncoder::new(&mut file).unwrap();
+
+        let (width, height) = decoder.dimensions().unwrap();
+        tiff.write_image::<colortype::Gray64>(width, height, &image_data)
+            .unwrap();
+    }
+    file.seek(SeekFrom::Start(0)).unwrap();
+    {
+        let mut decoder = Decoder::new(&mut file).unwrap();
+        if let DecodingResult::U64(img_res) = decoder.read_image().unwrap() {
+            assert_eq!(image_data, img_res);
+        } else {
+            panic!("Wrong data type");
+        }
+    }
+}
+
+#[test]
+fn test_rgb_u64_roundtrip() {
+    let img_file = File::open("./tests/images/gradient-3c-64b.tiff").expect("Cannot find test image!");
+    let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
+    assert_eq!(decoder.colortype().unwrap(), ColorType::RGB(64));
+
+    let image_data = match decoder.read_image().unwrap() {
+        DecodingResult::U64(res) => res,
+        _ => panic!("Wrong data type"),
+    };
+
+    let mut file = tempfile::tempfile().unwrap();
+    {
+        let mut tiff = TiffEncoder::new(&mut file).unwrap();
+
+        let (width, height) = decoder.dimensions().unwrap();
+        tiff.write_image::<colortype::RGB64>(width, height, &image_data)
+            .unwrap();
+    }
+    file.seek(SeekFrom::Start(0)).unwrap();
+    {
+        let mut decoder = Decoder::new(&mut file).unwrap();
+        if let DecodingResult::U64(img_res) = decoder.read_image().unwrap() {
+            assert_eq!(image_data, img_res);
+        } else {
+            panic!("Wrong data type");
+        }
+    }
+}
+
 #[test]
 fn test_multiple_byte() {
     let mut data = Cursor::new(Vec::new());