Fix viewer issue with Tower and Chase View.

Fix viewer issue with Tower and Chase View.
When looking _at_ a model with an x,y offset of 0,
then the view heading has no effect. So, force heading offset property
to 0 to keep other property consumers from running incorrect calculations.
=> Trying to rotate the heading offset in Tower/Chase view no longer
rotates the blue sky around the aircraft, though the viewer itself
isn't rotating anything.