Fix for #2009 (empty rectangle issue)

[Lehonti]

Closes #2009

Notice that OnMouseDown rounds the coordinates and OnMouseUp doesn't, so when testing for equality the coordinates are different, which shouldn't happen

[cameronwhite]

Yeah this definitely contributes to the issue - we should have a common function for doing this rounding (it also happens inside the handle for the OnMouseMove event, which is inconsistent )

In my testing I was still able to occasionally get it to produce a zero-size rectangle, by tapping my laptop's trackpad while zoomed in.
After adding some printouts the (before rounding) mouse up position is slightly different, and there wasn't a mouse move event.

down 476.6666666666667, 246.66666666666666
up 476.6666666666667, 246.33333333333334

The logic in HasDragged for transforming to view coordinates is suspicious, because when you're zoomed in you can move the mouse by one unit in view space, and still end up with a zero size rectangle in canvas space

[Lehonti]

@cameronwhite I see. Maybe after these new changes it will work better

[cameronwhite]

Thanks! Having the common function is much nicer

I can still reproduce the same issue though so I think it's somewhat separate from the rounding - being zoomed in far makes it much easier to hit.
I can do some more investigation, but from the printouts I added yesterday it seemed that the issue is that the mouse-up event might be at a slightly different location without reporting that first as a mouse move (and this might be different enough to round to a different canvas coordinate). So HasDragged() can return true (especially when zoomed since it does its comparison in screen coordinates, rather than canvas coordinates) but we don't actually expand the initial empty rectangle with the new position from the mouse up event.

Maybe this needs to be handled at a lower level like BaseTool or the canvas event handlers though. Very likely there are other tools which also have similar assumptions that the "mouse up" event happens at the same position as the last "mouse move" event, and sending out an extra "mouse move" event could work?