[Kst] treatment of invalid data in KstImage

[Ted Kisner]

In KstMatrix, it is possible for some elements to be infinite or NAN.  
Currently, KstImage draws these matrix elements as the first color in the 
palette.  This is confusing since invalid data will then have the same color 
as normal data that is simply off-scale.

I think that the correct thing to do is to paint these pixels the current 
background color (KstSettings::globalSettings()->backgroundColor).  
Unfortunately this class is compiled into libkstapp, and I need to access it 
from KstImage.cpp, which is in libkstmath.  

What is the most technically correct way to make KstSettings available 
globally?

-Ted