This should make such visual elements more visible on top of the document content.  Backend:

* NOT operations do not require a color since they ignore the source pixels

* All operations need 2 color hints for stippling

* Tweak color hints so that things are clear and look good

* Assert-fail in some raster-simluating methods if transparent colors are
  passed (not supported yet due to unprotected call to kpColor::toQColor())

* Comments, style

svn path=/trunk/KDE/kdegraphics/kolourpaint/; revision=700213