Use gimp_image_{freeze,thaw}_layers(), added in the previous
commit, to suppress updates to the Layers dialog while constructing
the decomposed layers, which significantly speeds up the operation
if the dialog is mapped.
(AKA Issue with grain merge layer mode)
Fall back to using the legacy grain-extract/merge modes when the
input image precision is integer, and only use the non-legacy modes
when the precision is floating point. The midpoint intensity value
of the non-legacy modes is 0.5, which isn't representable exactly
using integer precision, causing small deviations between the input
image and the reconstructed image.
Add an option to keep the decomposition in a layer group.
Add an option to add layer mask to each scales layers.
Do not use 'new from visible' because it produces unexpected result when
- image is already composed by several layers
- target layer has not the same size as the image canvas
Replaced by succession of layer copy and merge down.
Simon Budig