EddyEditor: trile selection can leak behind visible triles

While having a trile already selected, clicking another on a same plane where selection is can lead to it "leaking" and selecting a trile behind it. Presumably it's caused by the "select next one behind" behaviour we've implemented some time ago.

https://github.com/user-attachments/assets/86114000-af24-4779-861c-22347ea5795e