Use URLs directly for layout images

[alexcjohnson]

The data uri is necessary for export only, which should always use staticPlot - so if we're not in staticPlot mode, skip this conversion. Hoping this will improve the experience for plots that make heavy & dynamic use of layout images, by keeping the plotting process synchronous - which I'm calling a bug because we've seen the display become glitchy during pan/zoom in these cases.

I'll have a couple of tests to adapt as well as a changelog entry to add... but first I'm going to use the circleci build for testing in our application.

[archmoj]

@alexcjohnson Thanks for the PR. LGTM.
Please add a draftlog.
Also possibly investigate these failures: https://app.circleci.com/pipelines/github/plotly/plotly.js/11406/workflows/ea736858-eed0-4644-bbac-f80d66624105/jobs/252337/parallel-runs/9/steps/9-105

Concerning the image test failures we did address the problem on master in #7178; so you could skip them.

[archmoj]

Please fetch upstream/release-v2.35.3 branch and merge it into this branch.
That would fix the image tests.
Thank you!

[alexcjohnson]

@archmoj one of the tests that fails with my change is should remove the image tag if an invalid source. Instead we see the browser's "broken image" icon if it fails to load. I think this is useful behavior for chart creators so I'm inclined to leave it this way and adapt the test (run as-is in staticPlot mode but verify that the image points to the source unchanged in interactive mode), do you agree or do you want to call this a breaking change that I should fix?

Here's me intentionally breaking the first image of the layout-images mock:

FYI we've demonstrated that this greatly improves the experience for our tile-server-in-a-graph app - using relayout data to decide which images to show, so overall it's definitely a win!

[archmoj]

💃