widget: use reliable text direction checks, not heuristics

This commit switches the way in which the editor and helper functions check
for RTL text from a heuristic to using the actual text direction.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/editor.go

@@ -794,9 +794,6 @@ func (e *Editor) PaintText(gtx layout.Context) {
 		start, end := clipLine(e.lines, e.Alignment, e.viewSize.X, cl, pos)
 		line := e.lines[start.lineCol.Y]
 		off := image.Point{X: start.x.Floor(), Y: start.y}.Sub(scroll)
-		if start.lineCol.X > end.lineCol.X {
-			start, end = end, start
-		}
 		l := subLayout(line, start, end)
 
 		t := op.Offset(off).Push(gtx.Ops)

widget/label.go

@@ -68,13 +68,9 @@ func clipLine(lines []text.Line, alignment text.Alignment, width int, clip image
 }
 
 func subLayout(line text.Line, start, end combinedPos) text.Layout {
-	if start.lineCol.X == line.Layout.Runes.Count {
-		return text.Layout{}
-	}
-
 	startCluster := clusterIndexFor(line, start.lineCol.X, start.clusterIndex)
 	endCluster := clusterIndexFor(line, end.lineCol.X, end.clusterIndex)
-	if startCluster > endCluster {
+	if line.Layout.Direction.Progression() == system.TowardOrigin {
 		startCluster, endCluster = endCluster, startCluster
 	}
 	return line.Layout.Slice(startCluster, endCluster)