Add env var to disable automatic WebVisualizer use in Jupyter environment

[gutnar]

Add checking DISABLE_OPEN3D_WEB_VISUALIZER environment variable to disable automatically enabling web visualizer when in a Jupyter environment.

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

The problem is that when using the Jupyter extension in Visual Studio Code then the web visualizer does not function. This change allows to set an environment variable so that the web visualizer would not automatically be enabled.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

When using the gui.Application functionality and running it using the Jupyter extension in Visual Studio Code then no visualization appears. When setting the env var DISABLE_OPEN3D_WEB_VISUALIZER=True then the WebVisualizer is not enabled and an application window opens.

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ssheorey]

Thanks @gutnar for this useful update. I added docs, allowed lower case env var and made OPEN3D the prefix for the env var.