Added test of ToU8ForWriting. Found a bug.

jxl/src/image.rs

@@ -189,7 +189,7 @@ pub mod debug_tools {
 
     impl ToU8ForWriting for u16 {
         fn to_u8_for_writing(self) -> u8 {
-            (self as u32 * 255 / 65535) as u8
+            (self as u32 >> 8) as u8
         }
     }
 
@@ -255,4 +255,14 @@ mod test {
         assert!(image.as_rect().to_pgm().starts_with(b"P5\n32 32\n255\n"));
         Ok(())
     }
+
+    #[cfg(feature = "debug_tools")]
+    #[test]
+    fn to_u8() {
+        use super::debug_tools::ToU8ForWriting;
+        assert!(0xd7a0u16.to_u8_for_writing() == 0xd7);
+        assert!(0.5f32.to_u8_for_writing() == 0x80);
+        assert!(0xd7a02953u32.to_u8_for_writing() == 0xd7);
+        assert!((half::f16::from_f32(0.5)).to_u8_for_writing() == 0x80);
+    }
 }