transform polygone: Do not clear poly_in before transforming points

[Fellfalla]

Before this MR, doTransform(my_poly, my_poly, transform) did not work as it does for other messages, e.g. pose.

If this is the desired behavior should be discussed.

[ahcorde]

CI is not compiling https://build.ros2.org/job/Rpr__geometry2__ubuntu_noble_amd64/97/console

[CursedRock17]

I have left a change that should resolve the bug fix, while implementing this feature, without adding extra bloat. Branch reference for a quick test of the code that works for me.

[CursedRock17]

Suggested change

	std::vector<geometry_msgs::msg::Point32> points_transformed(poly_in.points.size(), {});
	for (size_t i=0; i < poly_in.points.size(); ++i) {
	doTransform(poly_in.points[i], points_transformed[i], transform);
	}
	poly_out.points = points_transformed;
	poly_out.points.resize(poly_in.points.size());
	for (size_t i = 0; i < poly_in.points.size(); ++i) {
	doTransform(poly_in.points[i], poly_out.points[i], transform);
	}

The tf2::Polygon cannot handle the = assignment cleanly, so the corresponding test fails. But if we want, an easy fix is to just call reserve on poly_out before we run the doTransform loop, then it will ensure there's space for the referenced point to be moved into the function call. This is beneficial since we're not copying any values.