Fixes bug #1792.
There are a couple of changes, here.  First, I changed the
DVisualLexicon for ding so that the default CustomGraphics size is 0.0,
not 50.0.  This is used as a flag in DNodeView that tells it to ignore
the default size.  I also changed the scaling so that it does not try to
rescale images to fit the width of nodes since in general we don't want
to change the user's aspect ratio.  It now scales images to fit within
nodes using the same width and height scaling.

Change-Id: If86cab41f84c042768131b3e5aa6260d4e8e5e97