Add graphicFill write support for polygon symbolizer (WIP)

[rdewit]

Description

The PR adds support for creating OL graphic fill styles from polygon symbolizer. Only types of Icon and Mark are supported.

It uses the newish renderer function for OL styles, which has one unfortunate drawback: it breaks the zIndex ordering and always draws the symbolizer on top of other ones.

Related issues or pull requests

This could solve #240 and #228

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Dependency updates
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe)
• I am unsure (we'll look into it together)

Do you introduce a breaking change?

• Yes
• No
• I am unsure (no worries, we'll find out)

Checklist

• I understand and agree that the changes in this PR will be licensed under the BSD 2-Clause License
• I have followed the guidelines for contributing
• The proposed change fits to the content of the code of conduct
• I have added or updated tests and documentation, and the test suite passes (run npm test locally)
• I'm lost; why do I have to check so many boxes? Please help!

[rdewit]

The PR is currently in draft mode as it's not complete (no tests yet) and may need further discussion about the implementation choices that I've made.

I tried to make the rendering as efficient as possible and I'd love to get feedback on how to improve this further, including how to better deal with scaled Icons.

[rdewit]

Thanks @jansule for getting GraphicFill support in (#388)! I'll close this PR.