Ensure canvas dialogs clean up all references to canvas ghosts on destruction

Nasal/canvas/api.nas

@@ -753,6 +753,14 @@ var Canvas = {
   getPath: func()
   {
     return "canvas://by-index/texture[" ~ me.texture.getIndex() ~ "]";
+  },
+  # Destructor
+  #
+  # releases associated canvas and makes this object unusable
+  del: func
+  {
+    me.texture.remove();
+    me.parents = nil; # ensure all ghosts get destroyed
   }
 };
 

Nasal/canvas/gui.nas

@@ -15,6 +15,13 @@ var Dialog = {
 
     return m;
   },
+  # Destructor
+  del: func
+  {
+    me.parents[1].del();
+    if( me["_canvas"] != nil )
+      me._canvas.del();
+  },
   # Create the canvas to be used for this dialog
   #
   # @return The new canvas