Input mouse cursor recall

A change in behaviour to what happens when mouse is set to invisible and shown again. Previously the mouse would be re-centered to prevent errors in virtual cursor mode, however as shown in the example provided (second commit) it is useful to be able to 'recall' the previous position to prevent a 'jump' in the cursor control.

There is currently no public 'SetMouseCursor' function (reasons given [here](http://urho3d.prophpbb.com/topic400.html) and [here](https://groups.google.com/forum/#!msg/urho3d/-VH3u2_sMHo/SscHwbuzVy0J) ), so this is one solution to keep in line with that.

An alternative solution might be to have a SetCursorVisible(bool, Cursor*), that way however your application controls the cursor, it can be restored to the cursor position.