core: Empty movie clips can not be used as a mask

When a movie clip or button is used as a mask, the masking will be
disabled if that object has no children; the maskee will be
completely visible. An empty movie clip inside an empty movie clip
successfully masks.

An EditText can also not be used as a masker (although it can be
wrapped inside a movie clip, and then the text successfully masks).

Add a `TDisplayObject::allow_mask` trait method that will
return whether the object can be used as a mask.

This fixes characters not being visible in Dad 'n' Me.

core/src/display_object.rs

@@ -786,6 +786,13 @@ pub trait TDisplayObject<'gc>: 'gc + Collect + Debug {
 
     fn instantiate(&self, gc_context: MutationContext<'gc, '_>) -> DisplayObject<'gc>;
     fn as_ptr(&self) -> *const DisplayObjectPtr;
+
+    /// Whether this object can be used as a mask.
+    /// If this returns false and this object is used as a mask, the mask will not be applied.
+    /// This is used by movie clips to disable the mask when there are no children, for example.
+    fn allow_as_mask(&self) -> bool {
+        true
+    }
 }
 
 pub enum DisplayObjectPtr {}
@@ -958,7 +965,7 @@ pub fn render_children<'gc>(
             context.renderer.pop_mask();
             clip_depth = clip_depth_stack.pop().unwrap();
         }
-        if child.clip_depth() > 0 {
+        if child.clip_depth() > 0 && child.allow_as_mask() {
             // Push and render the mask.
             clip_depth_stack.push(clip_depth);
             clip_depth = child.clip_depth();

core/src/display_object/button.rs

@@ -203,6 +203,10 @@ impl<'gc> TDisplayObject<'gc> for Button<'gc> {
     fn as_button(&self) -> Option<Self> {
         Some(*self)
     }
+
+    fn allow_as_mask(&self) -> bool {
+        !self.0.read().children.is_empty()
+    }
 }
 
 impl<'gc> ButtonData<'gc> {

core/src/display_object/edit_text.rs

@@ -169,6 +169,10 @@ impl<'gc> TDisplayObject<'gc> for EditText<'gc> {
         }
         context.transform_stack.pop();
     }
+
+    fn allow_as_mask(&self) -> bool {
+        false
+    }
 }
 
 unsafe impl<'gc> gc_arena::Collect for EditTextData<'gc> {

core/src/display_object/movie_clip.rs

@@ -387,6 +387,10 @@ impl<'gc> TDisplayObject<'gc> for MovieClip<'gc> {
         );
         self.set_removed(context.gc_context, true);
     }
+
+    fn allow_as_mask(&self) -> bool {
+        !self.0.read().children.is_empty()
+    }
 }
 
 unsafe impl<'gc> Collect for MovieClipData<'gc> {