Add an experimental sync to keep real time sync.

May help input, audio, and network related lag problems.
Make the lag sync adapt to minor variance.
Add UI for realtime sync option.
Fix lagsync with alternate speed set.
Avoid calling select() with an error pointlessly.

It's just a spin loop unfortunately anyway.
Merge pull request #6220 from unknownbrackets/lagsync

Add option for less timing lag (spin on Windows)