Make navmesh rasterization errors more lenient

Make navmesh rasterization on the navigation regions and map more lenient by starting out with a lower internal cell scale by default and changing the merge error to just warning that can be toggled.

modules/navigation_2d/nav_map_2d.cpp

@@ -80,7 +80,7 @@ void NavMap2D::set_merge_rasterizer_cell_scale(float p_value) {
 	if (merge_rasterizer_cell_scale == p_value) {
 		return;
 	}
-	merge_rasterizer_cell_scale = MAX(p_value, NavigationDefaults2D::NAV_MESH_CELL_SIZE_MIN);
+	merge_rasterizer_cell_scale = MAX(MIN(p_value, 0.1), NavigationDefaults2D::NAV_MESH_CELL_SIZE_MIN);
 	_update_merge_rasterizer_cell_dimensions();
 	map_settings_dirty = true;
 }