Fixed ProgressBar scrolling behaviour

[cansik]

As mentioned in #6174 the ProgressBar ist not scrolling as expected. This subtracts the necessary ImGui::GetScrollY() to the y coordinate.

Type

• Bug fix (non-breaking change which fixes an issue): Fixes ProgressBar widget is not scrolling within ScrollableVert #6174

Motivation and Context

Otherwise the ProgressBar is not usable within ScrollableVert.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

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This change is 

[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.

[cansik]

@ssheorey Thanks for looking into the PR, if something is missing, please tell me. Otherwise I would be happy if you could merge this and #6178 before the next release of Open3D. These two bugs are really breaking any serious GUI (even the official Open3D viewer does not work on MacOS).

[reyanshsolis]

@cansik Thanks for your valuable contribution. Is it possible for you to share a before-after screenshot/video for this PR ?

[errissa]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @cansik)