Expand gallery example "Focal mechanisms" to use "*fill" and "pen"

[yvonnefroehlich]

Description of proposed changes

This PR aims to extend the gallery example Focal mechanisms to use the parameters compressionfill, extensionfill, and pen added in PR #2345.

Preview: https://pygmt-dev--2433.org.readthedocs.build/en/2433/gallery/seismology/meca.html

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

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