The round corners script was assuming that the drawable always
was a layer, which is incorrect when a layer mask is selected.
We fix this by always requesting the active layer instead of
active drawable.
We also enable this script for RGB and GRAY with alpha
channel. Although it might not always make sense to
use this script when an alpha channel is present, there
can be use cases where it is perfectly fine.
So let the designer decide if this script is suited or not
instead of GIMP making that judgement.
(cherry picked from commit ed35579861)
# Conflicts:
# plug-ins/script-fu/scripts/round-corners.scm
b2fa1bd948