fix(canvas): remove __painter from Layer and inject painter in Painter#insertLayer when it doesn't exist

Author: plainheart

src/canvas/Layer.ts

@@ -89,8 +89,6 @@ export default class Layer extends Eventful {
 
     private _paintRects: BoundingRect[]
 
-    __painter: CanvasPainter
-
     __dirty = true
     __firstTimePaint = true
 
@@ -449,9 +447,8 @@ export default class Layer extends Eventful {
                     clearColorGradientOrPattern = createCanvasPattern(
                         ctx, clearColor, {
                             dirty() {
-                                // TODO
                                 self.setUnpainted();
-                                self.__painter.refresh();
+                                self.painter.refresh();
                             }
                         }
                     );

src/canvas/Painter.ts

@@ -597,7 +597,7 @@ export default class CanvasPainter implements PainterBase {
 
         layersMap[zlevel] = layer;
 
-        // Vitual layer will not directly show on the screen.
+        // Virtual layer will not directly show on the screen.
         // (It can be a WebGL layer and assigned to a ZRImage element)
         // But it still under management of zrender.
         if (!layer.virtual) {
@@ -623,7 +623,7 @@ export default class CanvasPainter implements PainterBase {
             }
         }
 
-        layer.__painter = this;
+        layer.painter || (layer.painter = this);
     }
 
     // Iterate each layer