libs: wayland: display: only handle the first output
libs: wayland: display: only handle the first output

Right now, all outputs are handled. The means that the registry object for
all but the last are leaked. As a result the sizes are not used correctly.

With two outputs, at first the mode and physical size of the second output
are used. If the first output changes the mode, then the physical size of
the second output is used in combination with the resolution of the first
output. The resulting pixel aspect ratio is incorrect.

There seems to be no way to determine on which output the window is shown,
so just use the first one to get consistent results.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/341>

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi