Add frame timing utilities

[randomPoison]

• Add more stopwatches to polygon and the engine in order to better capture timing.
• Track statistics about frame timing in the main loop and print analysis on shutdown. This is a temporary measure but it'll help identify when there are performance regressions or when I need to start working on optimizations.
• Add empty example for bootstrap-gl and update empty example for gunship.
• Add statistical analysis utilities to stopwatch for use in various places.