Fixes to the timing system

There was a huge bug in the timing system that made the framerate without vSync on completely different than what was specified (see PolyCore.cpp).

Also, I noticed that the windows core used GetTickCount() for timing, which isn't very accurate. I updated it to use QueryPerformanceCounter() instead, and now all my animation is silky smooooooth! I hope everyone else on Windows notices the same improvements I did.