- fix dartdocs for addPerformanceOverlay
- make PerformanceOverlayLayer honour its x, y, and height.
- fix the y axis of PerformanceOverlayLayer to only show 3x16ms, since
  if it's more than 16ms it really doesn't matter what it is.
- make the label on the PerformanceOverlayLayer show the worst time on
  record not the instantaneous time.
- pin the fps to a maximum of 60Hz