[Docking] Can't move out the window on XNA sample

[mmttim]

We can't move a window out of the viewport in the XNA sample. Docking is working fine. I tried to find the issue in the renderer but haven't found the problem yet.

[mellinoe]

The XNA sample hasn't been updated with multi-viewport support, and I don't know if XNA has the necessary features to make it work. If somebody knows how that could be accomplished, I'd happily merge a pull request to that branch. In the meantime, the regular sample project should fully work with multiple viewports.

[andreakarasho]

Probably spawning multiple Game instances [?]