[RF] Fix filled plotting of RooCurve.
[RF] Fix filled plotting of RooCurve.

[RF-8932] When a RooCurve is plotted as a filled polygon, one point has
to be added at the lower left of the plot to close the polygon.
A few lines of code have been cleaned up and simplified.

olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,karies/root,olifre/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root