GetMainViewport returns a ImGuiViewportPtr that we didn't wrapped

[antongit]

@azeno
The GetMainViewport can be handy in getting the center of the imgui screen among other things, like here:
https://github.com/ocornut/imgui/blob/03d3343e215f78945d149621bd5655a95db59bfc/imgui_demo.cpp#L3516

But it returns ImGuiViewportPtr, that we didn't wrapped
I've tried forwarding and using it, but vvvv crashes, because it's a pointer and has other pointers, right? Has to be handled properly.