VideoBackends: Store a backbuffer 'scale'

This is a scaling factor, used for hi-dpi configurations.

Source/Core/Common/WindowSystemInfo.h

@@ -32,4 +32,7 @@ struct WindowSystemInfo
   // on the platform. e.g. HWND for Windows, Window for X11. If the surface is
   // set to nullptr, the video backend will run in headless mode.
   void* render_surface = nullptr;
+
+  // Scale of the render surface. For hidpi systems, this will be >1.
+  float render_surface_scale = 1.0f;
 };