Add option to fix the framerate to the native screen refresh rate

Useful on mobile devices where using non-standard framerates
looks absolutely horrible and screen refresh rates vary highly.

src/eventthread.h

@@ -187,6 +187,7 @@ struct RGSSThreadData
 
 	Vec2 sizeResoRatio;
 	Vec2i screenOffset;
+	const int refreshRate;
 
 	Config config;
 
@@ -196,12 +197,14 @@ struct RGSSThreadData
 	               const char *argv0,
 	               SDL_Window *window,
 	               ALCdevice *alcDev,
+	               int refreshRate,
 	               const Config& newconf)
 	    : ethread(ethread),
 	      argv0(argv0),
 	      window(window),
 	      alcDev(alcDev),
 	      sizeResoRatio(1, 1),
+	      refreshRate(refreshRate),
 	      config(newconf)
 	{}
 };