Allow android/iOS forms UrhoSurface to be hidden and shown.

[rdpeake]

If Forms surfaces are hidden or a another page is placed in front of them, the Urho Application keeps running but the graphics surface is never added back to the view tree. As that surface is static, i simply add it to the view is it already exists when the surface renderer creates the view.

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[xamarin-cla-bot]

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[nicholasfortier]

Bumping this pull request. Do you plan on adding this quick fix to the master branch soon?
Do i need to create a new pull-request with the same changes to have it merged?
Thanks