Use GJK algorithm to select elements on drag

[sprocketc]

We can use Gilbert-Johnson-Keerthi distance algorithm to determine if a shape is within the bounds of our select rectangle. We currently check against the bounds of the element, which is probably not accurate enough. We can still use that as a fallback method, or to filter out elements before running the accurate collision check. GJK algorithm is a fast way to test when convex shapes collide. You can check a really nice interactive explanation of the algorithm here.