LayerSurface: handle focus when switching outputs

Currently if a layer surface is focused and the user focuses a different
output the layer surface remains focused. However, updating focus on
layer surface unmap only considers seats that have the layer surface's
output focused.

To fix this there are 3 approaches I see:

1. Unfocus all layer surfaces on the old output when switching output
focus, focus any layer surfaces on the new output.

2. Disallow switching output focus while a layer surface is focused.

3. Stop caring about output focus when determining which layer surface
should gain/lose focus.

I've taken the 3rd option here as it is significantly simpler to
implement and maintain but still feels reasonably intuitive.

river/Seat.zig

@@ -152,6 +152,7 @@ pub fn focus(self: *Self, _target: ?*View) void {
     // While a layer surface is exclusively focused, views may not receive focus
     if (self.focused == .layer) {
         const wlr_layer_surface = self.focused.layer.wlr_layer_surface;
+        assert(wlr_layer_surface.mapped);
         if (wlr_layer_surface.current.keyboard_interactive == .exclusive and
             (wlr_layer_surface.current.layer == .top or wlr_layer_surface.current.layer == .overlay))
         {