Since we rely on a global object ctor to add a display output to the
list, it is not referenced from anywhere else, so does not get linked in
when building tests since most objects are pulled from a static library.
Another option would be to use --whole-archive to link to it.
we want to be able to access either all outputs, or the currently
selected one (if any, else return the top one, which ought to be the GUI
one if we have one).
Hopefully this will lead the way to adding support for things like
Wayland and Haiku graphics, cf. #56.
We define a display_output_base class that display backends
can derive from to implement display-specific calls.
