Remove the special clipping-mode handling for channels throughout
the transform (and drawable-filter) code, and rather use
gimp_item_get_clip(), added in the previous commit, instead.  As
mentioned in the previous commit, we only modify the clipping mode
in top-level code, while having lower-level code use the clipping
mode as-is.  This not only hides the actual clipping-mode logic
from the transform code, but, in particular, allows code performing
transformation internally to use arbitrary clipping modes.

Also, this commit fixes a bunch of PDB bugs all over the place :)