app: implement color-erase mode as a GimpOperationLayerMode blendfun
... and get rid of the dedicated op. This gives us support for all the blend/composite options for this mode. Rename COLOR_ERASE to COLOR_ERASE_LEGACY, with perceptual blending/ compositing and immutable everything, and add a new COLOR_ERASE mode, defaulting to linear blending/compositing, with mutable everything. Modify affected code.
Showing with 100 additions and 252 deletions