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.
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.
