Retain brightness with lighting layers

[pstadler]

Add guard RGBLIGHT_LAYERS_RETAIN_VAL to retain the currently used val (as returned by rgblight_get_val())
when applying lighting layers.

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[stale]

Thank you for your contribution!
This pull request has been automatically marked as stale because it has not had activity in the last 45 days. It will be closed in 30 days if no further activity occurs. Please feel free to give a status update now, or re-open when it's ready.
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[pstadler]

I‘d still like to get this merged.

[drashna]

This should target develop, though.
You can edit the title, and that should allow you to retarget.

[pstadler]

@drashna thanks for the review. changed target to develop.

[drashna]

@drashna thanks for the review. changed target to develop.

Awesome! Though, it looks like you have a merge conflict here. And one that looks like you'll need to rebase/merge locally to fix.

[pstadler]

will provide an update after rebasing later today. sorry, went under my radar.

[pstadler]

done

[drashna]

Thanks!