Some observations of issues with FLTK backend

[David Bateman]

To check the performance of the FLTK plotting I tried to run all of the 
demos in scripts/plot. I noticed a few issues.

* There was no text anywhere (title, axis, etc). I suppose this is 
related to the fonts issue Shai mentioned so I'll wait on this one
* images aren't rendered
* "demo colorbar" seg-faults Octave
* patches with facecolor of none don't render in color but rather BW and 
so contour3 and contour don't work
* plotyy seems broken and so pareto doesn't work either
* Some fltk functionality is not mlocked and so some like "clear all; 
plot(1:10)" will segfault

Apart from that it appears to be coming along nicely.

D.