feImage causes crash if target svg element is used

[Ruud van Eeghem]

When applying a custom filter, I choose the feImage effect. Then in the effect parameter tab I use the button "selected svg element". The excpected behaviour is that the filter uses the source svg element as an image and then copies it to the destination svg element (the one that gets the custom filter). However, under normal use, the element the custom filter is applied to will be the current selected element for the user. So if the user presses this button without first selecting a source svg element the effect crashes inkscape.

At the bare minimum the effect should have a check on the source svg element not being the same as the destination svg element.

A more proper implementation would grab the cursor and allow the user to dynamically select an object on the canvas.

[su_v (suv-lp)]

Crash per bug summary line is already tracked in:
* Bug #195320 “feImage crashes inkscape if filtered object is the same as referenced object”
  https://bugs.launchpad.net/inkscape/+bug/195320

Proposing to link as duplicate to bug #195320