shadows should be visible in entire scene

[kfarr]

in 3dstreet scenes larger than 50m streets the shadows will stop appearing. for large scenes this means most of the area does not have shadows.

user goal is to see shadows all the time.

possible methods:

• increase the area of the shadowbox / shadow light to cover more of the scene
• move the shadow light source along with the camera or otherwise adjust the shadowbox location based on camera position as to not render too large of a shadow area that is beyond a user's immediate field of view
• both -- slightly enlarge the area and also keep it centered relative to the camera

[kfarr]

see #449