Support CSS background property in labels/text renderer

[nyalldawson]

This commit adds support for the CSS background-color and background-image properties when used with HTML labels.

• Backgrounds are supported for block items (eg <div>) or inline items (eg <span>)
• For images, the CSS should be formatted as background-image: url(xx). It supports local file paths, http links, or base64 embedded content
• Backgrounds are not supported for curved text
• HTML backgrounds are always rendered above any background shape for the label, and below drop shadows/buffers

Sponsored by City of Freiburg im Breisgau

[qgis-bot]

@nyalldawson
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
Also, any commit having [needs-doc] or [Needs Documentation] in will see its message pushed to the issue, so please be as verbose as you can.

Thank you!

[qgis-bot]

@nyalldawson

This pull request has been tagged for the changelog.

• The description will be harvested so please provide a "nearly-ready" text for the final changelog
• If possible, add a nice illustration of the feature. Only the first one in the description will be harvested (GIF accepted as well)
• If you can, it's better to give credits to your sponsor, see below for different formats.

You can edit the description.

Format available for credits

• Funded by NAME
• Funded by URL
• Funded by NAME URL
• Sponsored by NAME
• Sponsored by URL
• Sponsored by NAME URL

Thank you!

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit ebd2120)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit ebd2120)

[DelazJ]

Backgrounds are supported for block items (eg
) or inline items (eg )

@nyalldawson you might want to escape the tags here.

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9401
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!