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: 297354f847 ("Remove unnecessary wl_display_dispatch calls")
Closes: https://github.com/swaywm/wlroots/issues/2386
This requires functions without a prototype definition to be static.
This allows to detect dead code, export less symbols and put shared
functions in headers.
This removes any assumptions about how the libdrm headers are installed,
and uses the pkg-config include directories as we're "supposed to".
This only adds a partial dependency, since we don't actually need to
link against libdrm.
