Menu icons are blurry when the UI scale factor is > 1

When GTK's UI scaling is set to anything other than 1x, Garcon's menu icons appear blurry. To fix this, icons need to be rendered with their width/height multiplied by the UI scale factor, and then should be rendered to a cairo_surface_t with the scale factor taken into account. That surface can then be used to create the menu item's GtkImage.