transformation::operator+= is broken

The concatentation operator of the transformation object is broken. Simple test code:

```c++
#include

#include

namespace bp = boost::polygon;

int main() {
bp::transformation xform1(bp::axis_transformation::ATR::NULL_TRANSFORM);
bp::transformation xform2(bp::axis_transformation::ATR::ROTATE_LEFT);

bp::point_data shift;
xform1.get_translation(shift);

std::cout << "before: " << shift.x() << ", " << shift.y() << std::endl;
xform1 += xform2;
xform1.get_translation(shift);
std::cout << "after: " << shift.x() << ", " << shift.y() << std::endl;
}
```
The output for me is:
> before: 0, 0
> after: 32529, 852967680

Which is wrong, as the "after" values should be (0, 0) as well. If you look at the implementation in transform.hpp, it's seriously messed up:

```c++
// concatenate this with that
const transformation& operator+=(const transformation& tr) {
coordinate_type x, y;
transformation inv = inverse();
inv.transform(x, y);
p_.set(HORIZONTAL, p_.get(HORIZONTAL) + x);
p_.set(VERTICAL, p_.get(VERTICAL) + y);
// concatenate axis transforms
atr_ += tr.atr_;
return *this;
}
```

1. variables x and y are uninitialized, leading to garbage values.
2. the formula makes no sense; the translation of the tr object is not even used.

This seems like such an essential function to be screwed up. Did it just happened that this operator is not used anywhere else in the project?