Smudge: improvement & some fixes

Although premultiplied alpha channel is faster for some clip operations,
pixel level manipulations are more complex because transparencies
produce color loss.
Nevertheless, the current solution seems good enough for production.
1. The smudge is now more intuitive: default tool (finger) distorts
normally.
2. By pressing the ALT key (water drop), you get a smoother liquify.
Rremark: it's not appropiate for transparent areas (for example strokes
or airbrushes), due to premultiplied transparency color loss. The
complete solution seems not necessary at this moment.