widget: don't adjust selection in Editor.makeValid

Only rune positions are tracked for carets, and they only need adjusting
when changing Editor content, not just for re-layout.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/editor.go

@@ -240,8 +240,6 @@ func (e *Editor) makeValid() {
 		return
 	}
 	e.lines, e.dims = e.layoutText(e.shaper)
-	e.caret.start = e.closestPosition(combinedPos{runes: e.caret.start}).runes
-	e.caret.end = e.closestPosition(combinedPos{runes: e.caret.end}).runes
 	e.valid = true
 }