Fix ioq3 compatibility bug with lightgrid and only one light.

Fix ioq3 compatibility bug with lightgrid and only one light.

Correct math requires the ambient component of the lightgrid to be zero
in that case. However, ioq3 ignores lightgrid cells with all zero ambient
value, EVEN if the directed value is nonzero.

This change sets the ambient value to #010101 if it'd be pitch black. This
should be a minimal change without affecting light hue that fixes
lightgrid rendering. In engines like DarkPlaces, almost no map should
look different from this.

Fixes #137.