Feature: Lumination!

[JamzTheMan]

This enhancement adds a new property to lights called lumens.

A negative value, say -20, will subtract "light", therefore creating darkness. Lights are processed in absolute order, so lets say you have the following:

• Darkness Spell: lumens -20
• Deeper Darkness: lumens -100
• Torch: lumens 0 (default if not specified)
• Daylight Spell: lumens 60

(0) The torch light will first be drawn
(20) The darkness will then draw over the torch light (covering it up)
(60) Daylight will then draw over the darkness
(100) Finally, the Deeper Darkness will draw over everything else so far defined.