Add visual tests for primitive shapes

[khuntvidisha13]

What this PR does

Adds visual regression tests for basic Processing primitive shapes:

• rect()
• ellipse()
• triangle()
• arc()
• line()
• point()

How it works

Each test creates a baseline screenshot on first run and compares against it on subsequent runs.

Related

Extends the visual testing framework introduced in the visual-testing branch.

[dpoppe7]

Hello @khuntvidisha13! Thank you for your efforts, I'd like to leave a few comments on your PR.

Have you tried opening an issue first indicating the actually behavior, or missing test you found? It really helps maintainers understand the context and whether the approach fits the project's direction. It also gives a place to discuss the test structure before a lot of code is written.
You may want to visit these guidelines.

Since you are proposing test for different files under src/test/shapes, each of those would be much easier to review and merge as its own focused PR.
Perhaps, you could break this into smaller Issues/sub-issues/PRs - the title says "primitive shapes" but this touches blend modes, 3D shapes, strokes, colors, and transforms all at once, which makes it difficult to review and merge.

I'd recommend open one issue proposing visual regression testing as a concept, and get maintainer take :)