bug fix on comparing textSpan

Runtime/painting/basic_types.cs

     public enum RenderComparison {
         identical,
         metadata,
+        hoverCallback,
         paint,
         layout,
     }

Runtime/painting/text_span.cs

             RenderComparison result = Equals(this.recognizer, other.recognizer)
                 ? RenderComparison.identical
                 : RenderComparison.metadata;
+
+            if (!Equals(this.recognizer, other.recognizer)) {
+                result = RenderComparison.hoverCallback > result ? RenderComparison.hoverCallback : result;
+            }
+            
             if (this.style != null) {
                 var candidate = this.style.compareTo(other.style);
                 if (candidate > result) {

Runtime/rendering/paragraph.cs

                     case RenderComparison.identical:
                     case RenderComparison.metadata:
                         return;
+                    case RenderComparison.hoverCallback:
+                        this._textPainter.text = value;
+                        break;
                     case RenderComparison.paint:
                         this._textPainter.text = value;
                         this.markNeedsPaint();