Add support for user-selected and automatic frame skipping. Add support for PAL timings (20 ms per frame).

User-selected frameskip causes slowdowns if the game runs slower than the resulting frame rate, but synchronises correctly if the game runs faster.

Automatic frame skipping is still the default. It now only skips up to 8 frames, but in some games still skips that entire 8 frames. What's needed is an algorithm that averages frame latencies over a few seconds and skips while the latency is LOWER than the average.
Add a section about frame skipping in the readme.