Add a responsive coords scene to testbed_ui #21806
Description
What problem does this solve or what need does it fill?
The viewport_debug UI example doesn't work with web and isn't very useful as an example to users.
See issue: #21803
What solution would you like?
Remove the viewport_debug example, and replace it with a new scene added to the testbed_ui example that tests viewport coords.
Unlike in the viewport_debug example, there wouldn't be any need to switch between the viewport coords and px coords layouts. Correctness would be checked by the screenshot CI.
What alternative(s) have you considered?
We could improve the viewport_debug example and make it work on web.
I think it probably makes more sense for this to be a testbed_ui scene though.