Optimize getNextLineStartRect.

Runtime/painting/text_painter.cs

                 var prevCodeUnit = this._text.codeUnitAt(offset);
                 if (prevCodeUnit == null) // out of upper bounds
                 {
-                    var rectNextLine = this._paragraph.getNextLineStartRect();
-                    if (rectNextLine != null) {
-                        return new Offset(((TextBox) rectNextLine).start, ((TextBox) rectNextLine).top);
+                    var rectNextLineTop = this._paragraph.getNextLineStartRectTop();
+                    if (rectNextLineTop != null) {
+                        return new Offset(0, (float) rectNextLineTop);
                     }
                 }
             }

Runtime/ui/txt/paragraph.cs

             return result;
         }
 
-        public TextBox? getNextLineStartRect() {
+        public float? getNextLineStartRectTop() {
-            var top = (lineNumber > 0) ? this._lineHeights[lineNumber - 1] : 0;
-            var bottom = this._lineHeights[lineNumber];
-            return TextBox.fromLTBD(0, top, 0, bottom, TextDirection.ltr);
+            return lineNumber > 0 ? this._lineHeights[lineNumber - 1] : 0;
         }
 
         internal PositionWithAffinity getGlyphPositionAtCoordinate(float dx, float dy) {