Final Italian language file from Gio
Clarify 32bpp DIB conversion code when DIB is already 32bpp
Fix premultiplied alpha issues with various new tools

This was a severe bug, where new filters that lean on PD's central
compositor would crash on images with non-255 alpha values, because I
wasn't premultiplying alpha prior to invoking the compositor.

Many thanks to Gio for catching this.
Improve performance of manual 32bpp conversion