decided against z-levels without rebuilding the game from scratch @_@
cleaned up render_all a little to draw all backgrounds after computing them
cleaned up light computations into add_light and remove_light
added get_light, which does capping. This lets add and remove work properly without recomputing global light each time
removed dup code from add and remove light
changed light formula to have central light level be related to LSL, not always 100
fixed bug where light would darken region around it