GameINI: Fix GFXBackend crash

Fixes a regression from 5.0-12066, where setting the GFXBackend variable
to one other than the current global backend would crash Dolphin upon
launching the game.

Source/Core/Core/Core.cpp

@@ -242,6 +242,9 @@ bool Init(std::unique_ptr<BootParameters> boot, const WindowSystemInfo& wsi)
 
   Host_UpdateMainFrame();  // Disable any menus or buttons at boot
 
+  // Manually reactivate the video backend in case a GameINI overrides the video backend setting.
+  VideoBackendBase::PopulateBackendInfo();
+
   // Issue any API calls which must occur on the main thread for the graphics backend.
   WindowSystemInfo prepared_wsi(wsi);
   g_video_backend->PrepareWindow(prepared_wsi);