#19 Shared Collision Shapes

[Matt Wash]

Rigid bodies can share collision shapes that are the same shape to save memory. all cubes, for example, could use the same box collision shape as they have the same relative dimensions. Currently a new collision shape is created for each object. Ideally we would keep a map of existing collision shapes and re-use them when appropriate (well that's how i did it in my middleware proj). We could do without this but it would be a nice optimization.