PNG-Transformation and masks

[Winfried]

If I create a mask with the Amiunt Control button within teh PNG-Transformation,
the Flood Fill tool an the Curve Tool don't work. Using the Curve Tool the histogram of the output images is shown.
Maybe just mixed up input and output.
No problems if I create a mask with the Mask Transformation and use this mask with the PNG transformation.

[jsachs]

It turns out that all the masks were incorrectly being based on the output image instead of the input image, and the Transparent PNG transformation just showed the problem more than other transformations. Fixed for next release.