More robust fix for the "this.canvas" undefined situation

[asturur]

Bug introduced with #2083 and partially fixed with #2120.

to be discussed, do not merge.

[sapics]

@asturur Thank you for quick fix!

Sorry, I know little about transform.

If the purpose is to avoid undefined, getCanvas function would be better to

return (this.group && this.group.getCanvas()) || this.canvas

I am afraid that, if we need to know ctx is come from which canvas, we need more complex function like below (sorry, i do not know which canvas element would be transformed, this is just a example).

getCanvas(ctx.canvas);
...
getCanvas: function(canvasEl) {
  if (canvasEl && this.canvas) {
    if((this.canvas.upperCanvasEl && this.canvas.upperCanvasEl === canvasEl)
       || (this.canvas.lowerCanvasEl && this.canvas.lowerCanvasEl === canvasEl)
       || (this.canvas.cacheCanvasEl && this.canvas.cacheCanvasEl === canvasEl)) {
      return this.canvas
    }
  }
  return this.group && this.group.getCanvas(canvasEl)
  // or return (this.group && this.group.getCanvas(canvasEl)) || this.canvas
},

[asturur]

i do not need to know the canvas element.
I need just the canvas object to know the preserveObjectStacking parameter
I ended up doing another thing, simpler, that is ensure that every object on canvas HAS is .canvas property set.