Issue #12166: Fix split preview when different layer selected
This patch attempts to resolve issue #12166 (closed)
With NDE, we can now edit a filter when another layer is selected. This
broke an assumption in GimpFilterTool
that the selected layer is the same
as the layer the current filter is applied to.
This patch fixes the problem by checking if the filter is being editing, and then
using either the filter's drawable or tool->drawables->data
accordingly.