Make YSort stable

Keeps track of the order in which items are collected by
_collect_ysort_children, and uses that order to break
ties between items with similar Y positions.

(cherry picked from commit 8d3afa985b479417b46811050e9b8bd9f7861f06)

servers/visual/visual_server_canvas.cpp

@@ -62,6 +62,7 @@ void _collect_ysort_children(VisualServerCanvas::Item *p_canvas_item, Transform2
 				child_items[i]->ysort_xform = p_transform;
 				child_items[i]->ysort_pos = p_transform.xform(child_items[i]->xform.elements[2]);
 				child_items[i]->material_owner = child_items[i]->use_parent_material ? p_material_owner : NULL;
+				child_items[i]->ysort_index = r_index;
 			}
 
 			r_index++;

servers/visual/visual_server_canvas.h

@@ -52,6 +52,7 @@ public:
 		Color ysort_modulate;
 		Transform2D ysort_xform;
 		Vector2 ysort_pos;
+		int ysort_index;
 
 		Vector<Item *> child_items;
 
@@ -68,6 +69,7 @@ public:
 			ysort_children_count = -1;
 			ysort_xform = Transform2D();
 			ysort_pos = Vector2();
+			ysort_index = 0;
 		}
 	};
 
@@ -83,8 +85,9 @@ public:
 
 		_FORCE_INLINE_ bool operator()(const Item *p_left, const Item *p_right) const {
 
-			if (Math::is_equal_approx(p_left->ysort_pos.y, p_right->ysort_pos.y))
-				return p_left->ysort_pos.x < p_right->ysort_pos.x;
+			if (Math::is_equal_approx(p_left->ysort_pos.y, p_right->ysort_pos.y)) {
+				return p_left->ysort_index < p_right->ysort_index;
+			}
 
 			return p_left->ysort_pos.y < p_right->ysort_pos.y;
 		}