Graphics: Attempt to provide more consistent frame timings

We now actively track how far behind / in front of an
ideal timestep we are during each frame, and try to
catch up / delay approximate this timing.

Therefore we use more precise timers and sleep functions
(nanosleep if available). We also delay **before** the
final buffer swap so the frame displays at more consistent
points in time.

Not only should this provide a somewhat more consistent
looking map scrolling at lower frame rates, it also
guarantees that we don't fall out of sync eg. with the
Audio during longer cutscenes.

'Graphics.frameReset()' now finally has a function, in
that it resets the ideal timestep approximation, which I
beliefe was also its job in the original RMXP engine.

I'm not sure how well this will work when the frame rate
is set to the monitor refresh rate and vsync is turned on.
Very likely unnecessary frame skips will occur here and there
due to imprecise timers. In the future we should probably
check if the frame rate is equal to or higher than the
monitor rate, and disable frame skip accordingly.

These changes currently break the F2 FPS display (it shows
a value that's slightly too high).

src/eventthread.cpp

@@ -394,7 +394,7 @@ void EventThread::notifyFrame()
 
 	static uint64_t freq = SDL_GetPerformanceFrequency();
 
-	int32_t currFPS = freq / diff;
+	double currFPS = (double) freq / diff;
 	fps.acc += currFPS;
 	++fps.accDiv;