... drifts/sharpens when applying additional gradients

In GimpDrawableFilter, don't use gegl:over for operations without
an input pad, since this only works if the filter's mode is
REPLACE.  If the filter's mode is different, in particular, if it's
NORMAL, we end up compositing the operation's output against the
input twice: once in gegl:over, and again in the filter's
applicator.  Notably, this happens for the gradient tool.

Instead, revert commit 5b80d3d3
(with some additions to avoid constructing unnecessary nodes when
the operation has no input) and simply change the applicator's mode
to NORMAL if the oepration doesn't have an input, and the filter's
mode is REPLACE.