Fix debug renderEntities when entity's scroll is != 1

Without it, the debug rectangle has a wrong offset.
Added _world == null test for inline onCamera function
Added a cross method to Vector so you can get a cross product
Merge pull request #87 from julsam/debug-render

Fix debug renderEntities when entity's scroll is != 1
Merge pull request #88 from DjPale/upstream-master

Added _world == null test for inline onCamera function
Merge pull request #90 from thecodethinker/master

Simple Cross method in Vector class