Make 3D mode actually work again.

It appears it was broken (at least on Linux) because it "captures" the
mouse, by moving it to the center of the window every frame -- but this
movement caused a new mouse movement event to fire, so the event queue
was constantly full, and paint events never got through.

There was even code to fix this problem, but it only existed on an SFML
1.x compat codepath, and was deleted a few months ago.  Apparently it's
needed with SFML 2.0, too, at least with GTK.  Whoops.
Merge pull request #209 from eevee/fix-3d-mode

Make 3D mode actually work again.