alpha8() colorspace is very special in pigment. Firstly, it is not handled
by the lcms. More than that, most of the compositions do not work in it,
since it has a bit weird definition of what is color and what is alpha
channel. So when painting another colorspace above it, it is more sane
to do the composition in the source colorspace, and only then convert
the result to the destination alpha8.

This patch implements a property for a color space, which defines
whether the color space prefers such way of composition.

TODO: there is a small problem currently, the bitBlt function should
      request the compositeOp of another color space every time it is
      called. That is not ideal of course and should probably fixed with
      some caching strategy.