High-level pixel access methods

[Patrick Julien]

>
> > kubelka-monk colour strategy, I need at least six QUANTUM's, and possibly
> > more. And there may be places where it would be preferable to have
> > growing stack of colour data, for instance when mimicking glazes) So
> > making the pixeldata less dependent upon MAX_CHANNELS and the rest
> > of the code less dependent on there always being the same number of bytes
> > in a KisPixelData struct would be nice.)
>
> MAX_CHANNELS is internal to tyles... in your own painting code, it is not
> necessary, use imgSomethingIForgetDepth() to get the number of channels...
> This should really be part of KisPaintDevice.

For easier access, a new depth method has been added to KisPaintDevice.