examples/dmabuf-capture: add extra roundtrip for wl_output listener

This example was relying on wl_display_dispatch being enough to fetch output information. This worked by chance.

Add an explicit wl_display_roundtrip.

Other examples don't setup wl_output listeners, so they should be fine.

Fixes: 297354f8 ("Remove unnecessary wl_display_dispatch calls") Closes: https://github.com/swaywm/wlroots/issues/2386