First pass at adding expanded frametiming stats to the viewer.

[Geenz]

This adds the following stats:

• Frametime jitter cumulative
  • Accumulates jitter over the session. Probably not necessary. Included for now to experiment and better find a session-wide metric we can record.
• Frametime 99th percentile
  • 99th percentile of frametime over the last 5 seconds.
• Frametime 95th percentile
  • 95th percentile of frametime over the last 5 seconds.
• Frametime standard deviation
  • Standard deviation of frametime over the last 5 seconds.
• Frametime jitter 99th percentile
  • 99th percentile of frametime jitter over the last 5 seconds.
• Frametime jitter 95th percentile
  • 95th percentile of frametime jitter over the last 5 seconds.
• Frametime jitter standard deviation
  • Standard deviation of frametime jitter over the last 5 seconds.
• Frametime events
  • Number of times in the session that frametime has jumped a significant amount. 10% is the default.

Adds the following debug settings:
StatsFrametimeSampleSeconds - How long to collect samples to calculate the above timed metrics. Defaults to 5 seconds.
StatsFrametimeEventThreshold - The threshold in order to log a frametime event. Defaults to 10% (0.1).