When re-activating an operation tool by clicking on a different
drawable while the tool is active, we re-call the corresponding
procedure to re-activate the tool, which implictly initializes it.
Avoid initializaing it explicitly in addition to that, since this
leads to the creation of a new config object by the filter tool,
while the GUI still refers to the old, now-dead, config object,
causing CRITICALs or segfaults when changing any parameter.