SDL_RenderSetVSync(): Restrict vsync to 0 or 1

In the future, we might want to support special swap intervals. To
prevent applications from expecting nonzero values of vsync to be the
same as "on", fail with SDL_Unsupported() if the value passed is neither
0 nor 1.

src/render/SDL_render.c

@@ -4129,6 +4129,10 @@ SDL_RenderSetVSync(SDL_Renderer * renderer, int vsync)
 {
     CHECK_RENDERER_MAGIC(renderer, -1);
 
+    if (vsync != 0 && vsync != 1) {
+        return SDL_Unsupported();
+    }
+
     if (renderer->SetVSync) {
         return renderer->SetVSync(renderer, vsync);
     }