Ensure dragging selection is smooth

[Kully]

Issue

When you make quick mouse movements while dragging a selection, it is sometimes possible that the selection gets "stuck" near the edge of the canvas.

stuck.piece.mov

In terms of a solution, it would be best to allow relative mouse movements while dragging to translate in smooth movements of the selection, without getting cut off near the edge of the screen.

As part of the solution, and going all the way in terms of a complete solution, it might also make sense to simply allow the selection to travel outside of the canvas.