Activate possibility of missing-tiles below spawn

On the 4th and subsequent glitch-outs, missing tile glitches can occur
below the spawn tiles. This means that it should be theoretically
impossible to get stuck at the spawn point, surrounded by glitches,
incapable of dying
Merge pull request #31 from variousauthors/fix_interminable_glitches

Activate possibility of missing-tiles below spawn