Remove 3D mode

https://mail.gnome.org/archives/games-list/2014-June/msg00006.html

https://bugzilla.gnome.org/show_bug.cgi?id=720284
Make pause overlay transparent

The pieces are hidden anyway, so this results in us showing an empty
board.

https://bugzilla.gnome.org/show_bug.cgi?id=720284