output: destroy Layouts on Output removal

The Layout struct holds a pointer to the Output which becomes invalid
when the Output is destroyed so we must ensure all the layouts of an
Output are destroyed first.

river/Root.zig

@@ -206,6 +206,9 @@ pub fn removeOutput(self: *Self, output: *Output) void {
         }
     }
 
+    // Destroy all layouts of the output
+    while (output.layouts.first) |layout_node| layout_node.data.destroy();
+
     // Arrange the root in case evacuated views affect the layout
     fallback_output.arrangeViews();
     self.startTransaction();