add automatic polygonisation before accessing small transcription window
If the user has added a new line like I did here (the small interlinear mark between lines 1 and 2 in the right column), and then selects it for adding its transcription, the polygon is empty and one does not see anything in the small transcription window.
This should be recomputed before in such a moment, or the user needs a warning if he hasnt himself chosen to recompute.