Amount Mask toggle

[MarkT]

In PWP 7, if you Shift+click on the amount mask, the amount slider drops to 0 and the mask window turns black, and you can see the change in the preview window. In PWP8, this behaviour is not consistent. In some (Blur/Blur More), there does not appear to be anything happening with Shift+Click. In others (Gaussian) the mask window turns black, the preview effect is altered, but the slider does not move, and in others (Contour) the effect is altered in the preview window, but the mask window remains white and the slider does not move.

[jsachs]

I have fixed this for the next release. For the record, Blur/Blur More does work, but the effects are so subtle on large images that the difference is hard to see.