Possible perf issue due to unnecessary rendering of outputs when outputs are not visible

[DonJayamanne]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: N/A
• OS Version: N/A

Steps to Reproduce:

1. Install Jupyter Exetnsion
2. Create a notebook and run a cell with the following code (with a valid png file)

from IPython import display
display.Image("/<path>/output.png")

3. Copy the above cell along with the output and past this into the same notebook around 10-30 times or more
4. Close the notebook and re-open it
5. There are a lot of messages logged in the devtools console window about rendering image mime types (even when all outputs are not visible).

Rendering mimeType image/png Object
renderers.js:2 request Object
renderers.js:2 Rendering mimeType image/png Object
renderers.js:2 request Object
renderers.js:2 Rendering mimeType image/png Object
renderers.js:2 request Object

Note: At this point in time only a few cells are visible

In fact if you create a large code cell to occupy the whole notebook and reload you can still run into this same issue.

This leads me to believe theres a possible perf issue as we're rendering images when we should not.
Hence if we have a notebook with large outputs and a large number of them, then it will freeze or hang vscode.