Add basic shadow directional shadow mapping

[bdero]

• Nodes can be marked as "casting" and/or "receiving" shadows.
• For this initial implementation of the shadow caster, just allow for:
  • setting a single scene-wide shadow casting direction (as if there's a directional light in the scene)
  • setting a color multiplier applied to fragments occluded by shadows.

For this initial implementation, don't worry about self-intersecting mesh shadows.