Add a comment in Image.toByteData to limit more encoders

lib/ui/painting.dart

@@ -1574,6 +1574,10 @@ class Paint {
 
 /// The format in which image bytes should be returned when using
 /// [Image.toByteData].
+// We do not expect to add more encoding formats to the ImageByteFormat enum,
+// considering the binary size of the engine after LTO optimization. You can
+// use the third-party pure dart image library to encode other formats.
+// See: https://github.com/flutter/flutter/issues/16635 for more details.
 enum ImageByteFormat {
   /// Raw RGBA format.
   ///
@@ -1715,6 +1719,10 @@ class Image {
   ///
   /// Returns a future that completes with the binary image data or an error
   /// if encoding fails.
+  // We do not expect to add more encoding formats to the ImageByteFormat enum,
+  // considering the binary size of the engine after LTO optimization. You can
+  // use the third-party pure dart image library to encode other formats.
+  // See: https://github.com/flutter/flutter/issues/16635 for more details.
   Future<ByteData?> toByteData({ImageByteFormat format = ImageByteFormat.rawRgba}) {
     assert(!_disposed && !_image._disposed);
     return _image.toByteData(format: format);