Performance: 4x4 tracking square

[bryphe]

Discussion / suggestion from @jordwalke - we were thinking it'd be helpful to have a mode with a 4x4 tracking square moving 4 pixels every frame. This is useful especially when doing a 240fps recording - we can track dropped frames by seeing if the cube moved further than it should between frames.

This should be a mode / flag that could be enabled for performance testing.

[jordwalke]

Having a 4x4 square that moves 4 pixels each frame would be great (and there could be tick markers every 20px or something that helps you easily add up the milliseconds spent.