Fix #500 : the viewbox was set to 0 if one dimension was negative.
All that because of a comparison problem.
Related to #499 - removes arbitrary set in MarkGraphic#draw.
The new behviour is coherent with what is stated in the norm. Note that
the expected behaviour may change.
Related to #496. setStroke was not called on the good UomNode.
Fix the rendering of some MarkGraphic instances.
The objects returned by WellKnownName#getShape were sometimes invalid.
Indeed, Arc2D, Ellipse2D and Rectangle2D expect to receive positive
width and height. Considering symmerties in these objects, I've tried to
use Math.abs when it was possible. The leftmost x-ordinate and the angle
of the HalfCircle have received a special treatment.
Merge pull request #521 from agueganno/uom-problem

Small fixes in the renderer/legend.

Fixes #496. Fixes #499. Fixes #500.