Frontend: Support vsync for speed limiter

src/core/settings.h

@@ -13,6 +13,7 @@ struct Settings
   GPUBackend gpu_backend = GPUBackend::OpenGL;
   u32 gpu_resolution_scale = 1;
   u32 max_gpu_resolution_scale = 1;
+  bool gpu_vsync = true;
   
   // TODO: Controllers, memory cards, etc.
 };