core: Implement DefineButtonCxform SWF tag

(Although nothing uses this... it was only used in SWF version 2
and below!)

core/src/display_object/button.rs

@@ -89,6 +89,24 @@ impl<'gc> Button<'gc> {
         static_data.down_to_over_sound = sounds.down_to_over_sound;
         static_data.over_to_up_sound = sounds.over_to_up_sound;
     }
+
+    /// Handles the ancient DefineButtonCxform SWF tag.
+    /// Set the color transform for all children of each state.
+    pub fn set_colors(
+        self,
+        gc_context: MutationContext<'gc, '_>,
+        color_transforms: &[swf::ColorTransform],
+    ) {
+        let button = self.0.write(gc_context);
+        let mut static_data = button.static_data.write(gc_context);
+
+        // This tag isn't documented well in SWF19. It is only used in very old SWF<=2 content.
+        // It applies color transforms to every character in a button, in sequence(?).
+        for (record, color_transform) in static_data.records.iter_mut().zip(color_transforms.iter())
+        {
+            record.color_transform = color_transform.clone();
+        }
+    }
 }
 
 impl<'gc> TDisplayObject<'gc> for Button<'gc> {

core/src/display_object/movie_clip.rs

@@ -873,6 +873,7 @@ impl<'gc, 'a> MovieClipData<'gc> {
             TagCode::DefineBitsLossless2 => self.define_bits_lossless(context, reader, 2),
             TagCode::DefineButton => self.define_button_1(context, reader),
             TagCode::DefineButton2 => self.define_button_2(context, reader),
+            TagCode::DefineButtonCxform => self.define_button_cxform(context, reader, tag_len),
             TagCode::DefineButtonSound => self.define_button_sound(context, reader),
             TagCode::DefineEditText => self.define_edit_text(context, reader),
             TagCode::DefineFont => self.define_font_1(context, reader),
@@ -1240,6 +1241,32 @@ impl<'gc, 'a> MovieClipData<'gc> {
         Ok(())
     }
 
+    #[inline]
+    fn define_button_cxform(
+        &mut self,
+        context: &mut UpdateContext<'_, 'gc, '_>,
+        reader: &mut SwfStream<&'a [u8]>,
+        tag_len: usize,
+    ) -> DecodeResult {
+        let button_colors = reader.read_define_button_cxform(tag_len)?;
+        if let Some(button) = context.library.get_character_by_id(button_colors.id) {
+            if let Character::Button(button) = button {
+                button.set_colors(context.gc_context, &button_colors.color_transforms[..]);
+            } else {
+                log::warn!(
+                    "DefineButtonCxform: Tried to apply on non-button ID {}",
+                    button_colors.id
+                );
+            }
+        } else {
+            log::warn!(
+                "DefineButtonCxform: Character ID {} doesn't exist",
+                button_colors.id
+            );
+        }
+        Ok(())
+    }
+
     #[inline]
     fn define_button_sound(
         &mut self,