Add missing OpenGL error checks to draw funcs.
Improve testing procedure.

Combined the window_test and draw_test files into graphics_test.go, and
standardized the error results.
Add useless move method to Collision interface.
Convert physics system to use only pointers.

Also fixed a bug in Polygon creation that resulted in a malformed
bounding box.
Add missing documentation.
Split Renderable code from draw.go.
Remove NewCollision function.

The idea of making the Collision interface an all-encompassing object
kind of takes the point out of interfaces.
Add Mover interface.
Add missing documentation.
Add move method to Renderable object.

Also changed the Renderable object a bit to optimize movement.
Add Physics object.