Failed to execute 'drawImage' on 'CanvasRenderingContext2D' when Canvas instance is embedded in the ipywidgets layout

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For my project, it is required to embed Canvas from the ipympl backend in the ipywidgets Layout or AppLayout.
It worked perfectly with version 0.7.0, but with all newer versions the plot image is not rendered in the canvas with exception in Crome browser: `Error setting state: Failed to execute 'drawImage' on 'CanvasRenderingContext2D': The HTMLImageElement provided is in the 'broken' state. `

The minimum example:

```
import numpy as np
from matplotlib.figure import Figure
from ipympl.backend_nbagg import Canvas as Canvas, FigureManager

class PlotWidget(Canvas):

__num__ = 0

def __init__(self, **kwargs):
Canvas.__init__(self, Figure(dpi=100, figsize=(6, 5.5)))
self.resizable = False
self.header_visible=False

fig = self.figure
ax = fig.add_subplot(1, 1, 1)
self.lines, = ax.plot(np.linspace(0, 1, 100), np.linspace(0, 1, 100), 'o-', markersize=2, linewidth=2)

manager = FigureManager(self, self.__num__)
self.__num__+=1
self.draw()

import ipywidgets.widgets as widgets
from ipywidgets import AppLayout, Layout

class Layout(AppLayout):

def __init__(self, **kwargs):
AppLayout.__init__(self, **kwargs)
self.center = PlotWidget(**kwargs)
if "pane_widths" not in kwargs:
self.pane_widths = [1, '650px', '0px']
if "pane_heights" not in kwargs:
self.pane_heights = ['0px', '300px', '0px']

app = Layout()
display(app)

```

Start with the provided minimum example using ipympl's backend_nbagg Canvas inside an ipywidgets AppLayout, and reproduce the broken-image error in Chrome. Compare behavior between version 0.7.0 and newer versions; done means the plot renders in the embedded canvas without the drawImage exception.

Scritto dal modello di indicizzazione a partire dal testo della issue.