In gimp_drawable_get_active_mask(), when the drawable doesn't have
an alpha channel, set or clear the mask's alpha bit, according to
the state of the other bits, so that it never gets in the way of a
fully set/clear mask.  The value of the alpha bit doesn't matter
when there's no alpha channel, however, having a uniform mask
allows us to skip component masking altogether.

Additionally, provide a default implementation for
GimpDrawable::get_active_mask() which returns a full mask, and
remove the equivalent implementation for GimpChannel.