Computation of focus point is not fully correct

[hannobraun]

The focus point is the point on the model (in model coordinates) that the mouse cursor points at. This point is used for input, for example when rotating the model, it is rotated around the focus point.

This works mostly, but not fully. The focus point is not detected correctly when the cursor still points at the model, but just barely. Maybe the graphics and the focus point computations use slightly different perspectives, leading to some distortion. This also seems dependent on window size (or at least aspect ratio), as the effect is less pronounced when the window is square.

It might help to implement #17 first, to show what's going on.

[hannobraun]

I'd like to add to that that rotation seems a bit weird. It seems to me like it doesn't rotate around the focus point, but a point somewhat offset from that.

Not sure if that was broken from the beginning and I'm just noticing, or if I broke something in the last few days. I'm not really keen to look into it, as long as #17 remains unimplemented.

[hannobraun]

@chrisprice fixed the problem I described in the issue description in #614. The problem I described in my follow-up comment (the model not rotating around the focus point) is still present.