Allow to create GPUContextD3D12 with an existing device

[AmazingTurtle]

You're stating that Ultralight is designed to integrate well into existing game engines. However, looking at the GPUContextD3D12, you can only create a new device by passing a hwnd. I would love to see a constructor that allows you to pass an existing device - thats mostly the case when integrating into a game and would make things way easier. Also for me it'd be a hurdle to get into ultralig.ht.