set fmt and stride to private in ImageRgb

Signed-off-by: fufesou <shuanglongchen@yeah.net>

src/flutter.rs

@@ -242,7 +242,7 @@ impl VideoRenderer {
                     rgba.raw.len() as _,
                     rgba.w as _,
                     rgba.h as _,
-                    rgba.stride as _,
+                    rgba.stride() as _,
                 )
             };
         }