Add an in-game FPS overlay

[meekee7]

Frames per second (FPS) is a key performance indicator for a game. Looking at the FPS is a fast way to tell if a code change had a significant performance impact.
OpenTrespasser should have a built-in FPS indicator. It should at least show the current FPS.
Optionally it can show some advanced stats, like min/max FPS for the current session and average FPS for the current session and the past minute.