Add test for gray_i8_roundtrip

Check that an SBYTE (int8) image can be read and written to properly. The minisblack-1c-i8b.tiff image was created using the command `gdal_translate -ot Int8 -co PIXELTYPE=SIGNEDBYTE -scale 0 255 \-128 128 tests/images/minisblack-1c-8b.tiff tests/images/minisblack-1c-i8b.tiff`.

tests/encode_images.rs

@@ -188,6 +188,7 @@ macro_rules! test_roundtrip {
 }
 
 test_roundtrip!(test_u8_roundtrip, U8, u8);
+test_roundtrip!(test_i8_roundtrip, I8, i8);
 test_roundtrip!(test_u16_roundtrip, U16, u16);
 test_roundtrip!(test_i16_roundtrip, I16, i16);
 test_roundtrip!(test_u32_roundtrip, U32, u32);
@@ -200,6 +201,11 @@ fn test_gray_u8_roundtrip() {
     test_u8_roundtrip::<colortype::Gray8>("minisblack-1c-8b.tiff", ColorType::Gray(8));
 }
 
+#[test]
+fn test_gray_i8_roundtrip() {
+    test_i8_roundtrip::<colortype::GrayI8>("minisblack-1c-i8b.tiff", ColorType::Gray(8));
+}
+
 #[test]
 fn test_rgb_u8_roundtrip() {
     test_u8_roundtrip::<colortype::RGB8>("rgb-3c-8b.tiff", ColorType::RGB(8));