Check snapping tolerance of both layers before validating intersection snapping

Currently, when searching for intersection snapping, there is no check on the distance. If the angle between the two segment is small, they can both be within the snapping tolerance but intersect very far from the mouse: 
![snapping_intersection](https://cloud.githubusercontent.com/assets/537624/2983303/730a9186-dc21-11e3-92ff-c6f00d912095.png)

When this intersection point is outside the screen, it can be very confusing for the user.

With this pull request, once the intersection point is found, it is checked according the snapping tolerance of the layer of both segments and only accepted if it is within both.