Add a new GimpItem::get_clip() virtual function, and a
corresponding gimp_item_get_clip() function, which return the
actual clipping mode to be used when transforming (or applying a
filter to) a given item, given the original clipping mode.  This
applies only to whole-item transformations (i.e., when not creating
a floating selection), and should be used by the top-level code
applying the transformation, rather than by the actual
transformation code, so that the item can be transformed using a
different clipping mode internally.

Provide a default implementation that simply returns the input
clipping mode, and override for GimpChannel (to always return CLIP)
and for GimpVecotrs (to always return ADJUST).