Fix scale-rotate ROI handler to allow to use a random scale axis and not only x/y

This patch allow to use scale-rotate handler without orthogonal center.
```
    1-----2
    |     |
    |     |
    |     |
    4-----3
```
On my application the handler 1 use the center 3, the handler 2 use the center 4...
Without this patch it is not possible.

```
            self.addScaleRotateHandle(pos=(0, 1), center=(1, 0))
            self.addScaleRotateHandle(pos=(1, 0), center=(0, 1))
            self.addScaleRotateHandle(pos=(0, 0), center=(1, 1))
            self.addScaleRotateHandle(pos=(1, 1), center=(0, 0))
```

Unfortunatly i think this patch break the code relative to scaleSnap

Then are you interested by this patch? Else can we create another handler type too allow this kind of scale-rotation?

Thanks a lot