Bug: a88/bcac

If the display is exported as SVG via the FreeHEP library, anti-aliasing
is subsequently disabled (for both text and lines) in the main graph
display.

A very strange bug -- haven't figured out the cause yet. FreeHEP gets
its own Graphics2D object and I can't see how it could mess with the
rendering hints for the on-screen one. In any case, the rendering hints
by the paint() method on every redraw. Adding a
‘if (g instanceof SVGGraphics2D) return;’ to the start of paint()
squashes the bug (but of course results in a blank SVG); this implies
that it's the actual process of drawing to the FreeHEP graphics object
which disables anti-aliasing, rather than anything peripheral in the
saveToSvgFreehep method.

Maybe obvious in retrospect, but RenderingHints objects are not
immutable, and FreeHEP has a bug which wipes the entire RenderingHints
object passed to it. Since GraphDisplay caches a single instance
of PuffinRenderingHints (on the reasonable assumption that nobody is
going to change it), this same empty RenderingHints object is retained
and passed to the usual draw-to-screen Graphics2D once the SVG export
is complete.

Fixed by adding a dirty bit to PuffinRenderingHints so getInstance()
can instantiate a new, pristine object when necessary, and by 
getting rid of the RenderingHints cache in GraphDisplay.