Fixed painting on transparent layers

[Boudewijn Rempt]

I just fixed the 'normal' composition mode for painting (it was more like 
'composite behind' but not quite), and made the layer merge code use the 
composition method defined for the layer. Now I only have to add a combobox 
to the layer properties dialog to enable layer composition methods. And 
that'll mean, probably, that I will have to implement all the composition 
methods declared in kis_global...

I'm wondering -- whose code shall I nobble? ImageMagick is quite clear, and 
judging from the enum, Krita was based on its list of operations, but the 
Gimp's code supports more and more interesting operations, and the Gimp has
optimisation code for lots of processors. At the moment I feel I'd better 
stick with ImageMagick.

-- 
Boudewijn Rempt | http://www.valdyas.org/fading/index.cgi