wayland: Relax conditions in strut edge heuristics

When the output layout changes, it's possible that the bottom panel will float in the middle of the screen, e.g. left edge of the panel touches the left screen edge while top and bottom panel edges touch no screen edge.

In that case, XdgToplevelWindow::strutRect() will erroneously indicate that the left strut rect is valid, while it's not.

Since the strut area is garbage, Window::checkWorkspacePosition() may incorrectly move and resize windows during output layout change.

The proposed heuristic will fail with square panels, but those are rare and the only way to detect the correct strut area would be to use the layer shell protocol.