GameScreen.update

update's responsibility is to:

• Process player input
• Update game state - object positions, do collision detection, etc.
• Run any character AI
• Kick off any audio associated with changes in this update
• Transition to other screens if appropriate

It is NOT supposed to:

• draw - that's the job of drawFrame
• load files, bind textures, or similar - that's the job of load
• set uniforms or other OpenGL objects - do one-time things in load and per-frame things in drawFrame