[ent geo phys shell] Add Spatial class
[ent] Controllable uses Spatial
[ent] Fix double bullet firing
[ent shell] Add Agent class abstracting vision
[ent world] Robot inherits from Agent
[ent] Flyer compiles
[ent world] Add flyer resources
[ent] Fix Biped being unable to move due to overridden buttons variable
[ent shell world] Make Flyer show up properly
[shell] Add a Flyer to initial state
[ent] Flip Flyer horizontally
[ent shell] Fix some Flyer issues
[world] Fix Flyer restitution