GdkWaylandWindow: window_update_scale: Don't reset scale to 1 with no outputs

When there is on_monitors_changed called on GdkWindow which doesn't have any outputs on its impl->display_server.outputs list, the scale was being reset to 1, causing GtkPopovers to set their scale to 1 and having blurry appearance on hidpi screens.

Fixes #7 (closed)

This also allows phosh!531 to work properly.

nice find, is there an upstream MR that goes with that?

/cc @adrien.plazas

Upstream MR: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/2157

An alternative fix would be to make phoc send enter and leave signals on subsurfaces as well. Mutter and Weston do, but wlroots and KWin don't. [EDIT] Actually, sway does. It may make sense to fix it on both sides.

The fix makes sense to me, though I'd like to see it merged upstream before merging it here, unless any of you two think we should merge it nonetheless.

It also still makes sense to me, but at least it doesn't really make a difference for us anymore since the issue it's fixing doesn't happen now thanks to phoc!171, so at least it doesn't hurt to wait ;)