This patch introduces a new function "Layer::editin_mask_bounding_rect"
that is used within the LevelsDialog, Luminosity and Colormasking to
process only the area where a mask was applied. Therefore we can
greatly reduce the amount of processed pixels if only a small portion
of the image was masked.
This patch adds the ability to refine a editing mask by a color-range
based on the pixels of the content image. This is useful for image
editing where mask restirction via luminosity might not fit or just
certain color ranges should be edited.
This adds a function where editing masks can be refined by selecting
a luminosity range that is applied to the content image and mapped to
the editing mask. This function allows the editing of image regions
that match only certain luminosity values.
