EMI: GetActorPuckVector should return nil for actors that have never followed walkboxes.

In Lua, The wear chore of dumbshadow.cos is only started if GetActorPuckVector returns a non-nil value. In the original game GetActorPuckVector seems to always return nil until a call is made to SetActorFollowBoxes.

This fixes the candles not appearing in the set gmi. The original Lua code only starts the candle animation if the candle actor is not playing any chores. Previously the wear chore of dumbshadow.cos was playing, so the animation was never started.
EMI: Don't set global alpha to 0 when light mode is set to LIGHT_NONE.

Fixes sprites not appearing in various places. For example the lights on Stan's house now show up.
EMI: Add support for additively blended sprites.
EMI: Read vertex color values from sprite files.

This makes the coals in wed more transparent, matching the original game better.
EMI: Draw model faces with alpha textures also to the depth buffer. Fixes #874
EMI: When a chore is stopped, remove it immediately from the list of playing chores.

If the chore is not stopped immediately, its components will still be drawn in the next frame, which is wrong. This caused flicker when entering the closeup at Stan's house.
EMI: Sprite drawing fixes for OpenGL Shaders renderer.
EMI: Read depth write and alpha test flags from sprite files.