paint the interior of current drawing polygon

For better eye guidance, the interior of the current shape is painted and updated when moving mouse. This feature can be frequently found in other labeling tools.

labelme/widgets/canvas.py

@@ -486,6 +486,15 @@ class Canvas(QtWidgets.QWidget):
         if self.selectedShapeCopy:
             self.selectedShapeCopy.paint(p)
 
+        if (
+            self.createMode == 'polygon' and self.current is not None and
+            len(self.current.points) >= 2
+        ):
+            realTimeShape = self.current.copy()
+            realTimeShape.addPoint(self.line[1])
+            realTimeShape.fill = True
+            realTimeShape.paint(p)
+
         p.end()
 
     def transformPos(self, point):