In gimp_gegl_apply_cached_operation(), add a boolean
connect_src_buffer parameter, which determines whether to connect
the source buffer to the operation-node's input, or to use its
existing input.  In gimp_drawable_merge_filter(), pass FALSE for
connect_src_buffer, so that the existing filter-node input is used.
This produces an equivalent result, however, it avoids invalidating
the filter node, and dropping cached data as a result.  In
operations that cache larger areas than the ROI, this avoids
reprocessing already-cached data when processing the rest of the
operation.

Additionally, in gimp_gegl_apply_cached_operation(), use an empty
input for the operation if src_buffer is NULL and
connect_src_buffer is TRUE; previously, we'd use the operation-
node's existing input when src_buffer was NULL.  Furthermore, crop
the operation-node's input to the destination rect when crop_input
is TRUE, even if connect_src_buffer is FALSE.