add nvidia vsync hack from the SDL driver (copy and paste; not worth splitting it out)

stepmania/src/arch/LowLevelWindow/LowLevelWindow_X11.cpp

@@ -40,6 +40,18 @@ void *LowLevelWindow_X11::GetProcAddress(CString s)
 
 CString LowLevelWindow_X11::TryVideoMode(RageDisplay::VideoModeParams p, bool &bNewDeviceOut)
 {
+#if defined(LINUX)
+	/* nVidia cards:
+	 *
+	 * This only works the first time we set up a window; after that, the
+	 * drivers appear to cache the value, so you have to actually restart
+	 * the program to change it again. */
+	static char buf[128];
+	strcpy( buf, "__GL_SYNC_TO_VBLANK=" );
+	strcat( buf, p.vsync?"1":"0" );
+	putenv( buf );
+#endif
+
 	XSizeHints hints;
 	XEvent ev;
 	stack<XEvent> otherEvs;