fix(canvas): fix potential NPE in canvas pattern dirty callback handler

[plainheart]

As we discussed before, some extension libraries may create their own Layer for customization via Painter#insertLayer and probably won't inject the painter property into the canvas painter like echarts-gl and echarts-for-weixin [1], this results in a potential NPE in the canvas pattern dirty callback handler.

So it's better to remove the redundant __painter from Layer to reuse the existing painter property and inject painter in Painter#insertLayer when it doesn't exist.

[1] Error threw in echarts-for-weixin when loading the pattern