New feature for review

[LukasT.dev@gmail.com]

> > Here is the new patch [1] and now when you press Shift and drag with
> > mouse left/right the spray brush changes it's size. Change the cursor
> > mode to _brush outline_ to see changes.
> 
> Nice patch !
> It is both intuitive and fast. In my case, it's much better than using
> the mouse wheel, as it is usable with the stylus.

Cool!
 
> I think up/down should do the same, because instictively we try to
> resize the brush as a shape or windows (dragging the corner for 1:1
> proportions). Also, it would be too hard to move perfectly horizontally
> or vertically. The only exception would be for changing the ratio, but
> not all paintops have a ratio option.

You don't have to move perfectly. The code detects if your movement is more 
vertical then horizontal or wise-versa.