Asset and rendering update

Fixes bugs in asset utility method calls.  Starfall::Background has been
changed to Starfall::Parallax to make better sense and allow for a
Parallax object to be used in the background and foreground.  Foreground
parallax should use a negative distance.

This commit also improves rendering performance by deferring the drawing
of Parallax elements into the class itself; unlike normal assets, this
is dependent upon the graphics context and the camera coordinates.