scene: ensure layer surface usable area >0

Clients may, for example, commit an exclusive zone larger than the
output dimensions. Compositors must handle this gracefully, which likely
requires more work on the compositor side but returning a usable area
with negative width or height in wlr_scene_layer_surface_v1_configure()
is nonsensical.

types/scene/layer_shell_v1.c

@@ -70,6 +70,13 @@ static void layer_surface_exclusive_zone(
 		usable_area->width -= state->exclusive_zone + state->margin.right;
 		break;
 	}
+
+	if (usable_area->width < 0) {
+		usable_area->width = 0;
+	}
+	if (usable_area->height < 0) {
+		usable_area->height = 0;
+	}
 }
 
 void wlr_scene_layer_surface_v1_configure(