fillOpacity support for the markSymbolizer geostyler object

[pragmaeuge]

Description

Hi, I have developed a simple mechanism for the support of the fillOpacity property in the markSymbolizer

Related issues or pull requests

At the moment, the "fillOpacity" of the markSymbolizer object is not supported. From now on, the "fillOpacity" property can be used to set the opacity property in the OpenLayer fill style characterization. However, the markSymbolizer "opacity" has the highest priority.

Pull request type

Please check the type of change your PR introduces:

• [ ] Bugfix
• [x] 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
• [x] I am unsure (no worries, we'll find out)

Checklist

• [x] I understand and agree that the changes in this PR will be licensed under the BSD 2-Clause License
• [x] I have followed the guidelines for contributing
• [x] 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!

[jansule]

Thanks @pragmaeuge

Could you clean up your commit history, so that only the relevant commits are included in this PR?

[KaiVolland]

@jansule we can squash and merge this commit to cleanup the history