Add to scene directly instead of items

The overlay was added to the game's items, causing it to be ticked
by voxel-engine. If the overlay was removed during the ticking loop,
an out of bounds array access would occur. Fix by adding overlay to
the game scene directly instead, since ticking isn't needed here.

Closes https://github.com/deathcap/voxpopuli/issues/18 TypeError: Cannot call method 'tick' of undefined