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);
 	}