Fix VSync renderer workaround

SDL just got an API to toggle VSync without having to tear down the
renderer ( libsdl-org/SDL#4157 ). We can remove the workaround and use
that instead. For now, we are putting it behind an ifdef until SDL
2.0.18 officially releases in November.

Fixes #831.

desktop_version/src/Screen.h

@@ -27,7 +27,7 @@ public:
     void toggleFullScreen(void);
     void toggleStretchMode(void);
     void toggleLinearFilter(void);
-    void resetRendererWorkaround(void);
+    void toggleVSync(void);
 
     bool isWindowed;
     bool isFiltered;