Fix saving an image as truecolor, on an 8-bit screen i.e. fix kpEffectReduceColors::convertImageDepth(32) [it turns out we never tested that code path, in KolourPaint/KDE4, so put back the TODO; KolourPaint/KDE3 is fine because there's a QImage::convertDepth() in Qt3, rather than QImage::convertToFormat()].  Internally, "24-bit" images have a depth of 32, in line with QImage.

Add plenty of comments and debug.

svn path=/trunk/KDE/kdegraphics/kolourpaint/; revision=725704