Painting on a temporary layer

[Bart Coppens]

Because painting with the filters invert and emboss didn't quite work the way 
I expected them to, I decided to let them paint on a temporary layer. While 
working it out I found out that the duplicate painting also needs this to work 
like other paint applications, so I made the change there as well. 
 
The code currently committed does the basic work, however I didn't activate it 
yet because I'm having a problem (actually 2, but the second should be more 
easily fixable). Whatever I try, when I paint or finish painting, the image 
doesn't get updated. I have to manually refresh the view to see the result. I 
tried making the temporary layer visible, setting it active, nothing seems to 
work. Maybe I'm forgetting something trivial to make it repaint? The code is 
in void KisToolFreehand::initPaint. 
 
Bart Coppens