Doomsday Engine

Map Renderer|Light Grid: Updated LightGrid implementation; addressed limitations

Revised the LightGrid, re-implementing it in C++ with Doomsday 2.0
components. The internal design of this component is mostly unchanged
however.

• The grid is initialized automatically should it be enabled after
  the map has already loaded.
• Significantly reduced memory requirements through the use of a sparse
  grid of block references and a special "null-block".
• An instance of LightGrid is now owned by the GameMap rather than a
  global "singleton".