[Improvement] Don't use GameWindow from OpenTK

[maskedByte]

Hi,

first of all, thank you for the great work you've done here.

In your implementation of DearImGui.OpenTK/ImGuiController.cs, you're using the GameWindow, which is not necessary because every functionality you're using comes from the NativeWindow class. So, it would be better to have this.

And of course, you will make me very happy ;)

Kind Regards
Tony