Faulty selection box behavior

[aganders3]

The selection box has an issue when it is activated (Shift key pressed) while the mouse button is already down.

To reproduce:

1. Hold Shift, then click, drag, and release to select some points. Then release Shift.
2. Begin rotating the camera (click + drag)
3. Without lifting the mouse button, hold shift
4. The camera will stop rotating, but no selection box is shown
5. Two issues may appear here:
   a. If you release Shift first the camera jumps on the next mouse move
   b. If you release the mouse button first, some unexpected points may be selected

We should think through the desired behavior for these edge cases prior to fixing.

[andy-sweet]

I think these were fixed in #88 . Reopen if you can still reproduce.