max_width was applied to the source box, but not to the cairo surface.
The cairo surface would therefore take on arbitrarily large dimensions
according to the required dimensions to fit the text input, which if
large enough would cause failures during output rendering and leave a
black hole in the titlebar.
When a floating container has a titlebar, render_backing_buffer()
ends up being called each time the container is moved. Add some
more checks for no-op changes in sway_text_node_set_max_width()
and sway_text_node_set_background(). This makes the move smoother.
