app: restore operation src node in gimp_gegl_apply_[cached_]operation()
When merging a drawable filter, we call gimp_gegl_apply_cached_operation() on a node that's part of the drawable's filter stack graph. The function rewires the node's input, and doesn't restore its original input connection before returning, leaving the graph in an inconsistent state. Currently, this doesn't matter, since we remove the filter right after that, but the next commit expects the filter stack graph to remain consistent. Remember the original source node of "operation" in gimp_gegl_apply_cached_operation(), and restore it upon exit, to fix that.