Implement image comparison for visual tests

[SableRaf]

Implement a visual snapshot testing system, inspired by the one used in p5.js.

[!NOTE] This has been added to our GSoC 2025 project list.

Goals

• [ ] Develop a visual snapshot testing framework for Processing.
• [ ] Automate image comparisons using pixel matching (e.g., image-comparison).
• [ ] Integrate the testing system into GitHub workflows.
• [ ] Port existing p5.js snapshot tests and develop new ones for Processing’s Java environment.

Motivation

This will benefit both the Processing community and its future contributors by providing a reliable way to catch visual regressions, making it easier for contributors to propose changes with confidence.

References

Read more about the p5.js approach in the documentation and this contributor blog post.