Bugfix: correction to Plot Options stretch function curve

[bmorris3]

Description

The Plot Options stretch histogram curve from #2390 looks fine in Gray colormap, but does not look fine in other colormaps. Here's an image with a linear stretch showing a nonlinear curve in the stretch histogram:

In reproducing the logic in glue's CompositeArray module, I apply the stretch, contrast, and bias correctly, but then I went one line too far and also applied the colormap to the values that should represent the curve. That's not what we want! In the gray colormap case, the colormap is linear+monotonic so you don't notice the problem.

This PR corrects the implementation to work with any colormap:

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)? 🐱

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files	Coverage Δ
...nfigs/default/plugins/plot_options/plot_options.py	91.29% <100.00%> (-0.71%)	⬇️

... and 3 files with indirect coverage changes

📢 Thoughts on this report? Let us know!.

[kecnry]

Glad to see it wasn't anything too involved - thanks!

[pllim]

Is there a way to add a regression test, to make sure a couple of stretch functions do not look weird when colormap is changed?