Fix for bug in 2D -> falling through tiles

Steps to reproduce:

Open demo "platformer" and add following properties to "player":

  * gravity: 1.9999897453
  * points: [[-5, -5], [5, -5], [5, 5], [-5, 5]]

Expected result: Changed properties of the player object

Actual result: Player falls through tiles
Merge pull request #22 from izidormatusov/master

Fix for bug in 2D -> falling through tiles