Fix SelectTool when zoomed in

[Rexbas]

As described in issue #269, the SelectTool does not highlight the right point when zoomed in, making it imposible to select the right point and move it. In issue #269 it was suggested to change the hitOptions tolerance to 1. However, a tolerance of 1 makes it harder to select points when not zoomed in. I have created a fix that still uses a tolerance of 10 but this scales with the zoom. So when zoomed in, the tolerance gets smaller. This makes it possible to select points when zoomed in.