Fix bug in Basis [] operator and add missing Transform-Vector3 * operator

The [] operator of Basis was returning a reference to a temporary, so fixed it.
There was no * operator in Transform equivalent to the xform function, which is
not in line with GDScript behavior.

Also fixed remaining cases where Transform relied on the old behavior of the
[] operator of Basis (i.e. that it returns the row, not the column).

include/core/Basis.hpp

@@ -307,7 +307,7 @@ public:
 
 	Basis();
 
-	const Vector3 &operator[](int axis) const {
+	const Vector3 operator[](int axis) const {
 		return get_axis(axis);
 	}
 

include/core/Transform.hpp

@@ -58,6 +58,13 @@ public:
 	void operator*=(const Transform &p_transform);
 	Transform operator*(const Transform &p_transform) const;
 
+	inline Vector3 operator*(const Vector3 &p_vector) const {
+		return Vector3(
+				basis.elements[0].dot(p_vector) + origin.x,
+				basis.elements[1].dot(p_vector) + origin.y,
+				basis.elements[2].dot(p_vector) + origin.z);
+	}
+
 	Transform interpolate_with(const Transform &p_transform, real_t p_c) const;
 
 	Transform inverse_xform(const Transform &t) const;