Compares its two arguments for order. Returns a negative integer, zero,
or a positive integer as the first argument is less than (closer to the viewer),
equal to, or greater than (further from the viewer) the second argument.
The compare method will be called with 2 objects of type
TransparencySortGeom and it's result should indicate which object is
closer to the viewer. Object1 < Object2 if it is to be considered closer
and rendered after.