Add visual regression tests to visual-testing branch

Summary

The visual-testing branch has a testing framework but several areas
lack test coverage. I'd like to add visual regression tests for:

• Primitive shapes (rect, ellipse, triangle, arc, line, point)
• Blend modes (OVERLAY, HARD_LIGHT, SOFT_LIGHT)
• Stroke styles (weight, cap, join)
• Transform operations (translate, rotate, scale)

Approach

Each area would be a separate focused PR.

Related

PR #1460 (to be broken into smaller PRs)

首先检查 visual-testing 分支现有的测试框架和 PR #1460。为所列出的图元、混合模式、描边样式和变换操作添加有针对性的视觉回归测试覆盖，并将每个领域作为单独的 PR 处理，然后验证测试通过。