circ from FastDelaunay.h depends on points ordering

It seems that this function works as intended for counterclockwise orders of points

```C++
vector<P> u;

u.push_back(P(2, 10));
u.push_back(P(3, 14));
u.push_back(P(4, 5));
u.push_back(P(5, 5));

circ(u[3], u[0], u[1], u[2]) // 0
circ(u[3], u[2], u[1], u[0]) // 1
```

Simple fix: add this before `p2 = ...`
```C++
if(a.cross(b, c)<0)
	swap(a, c); // Any swap will do
```