[BUG] image comparison diff detection and threshold unintuitiveness

[dimkin-eu]

Context:

• Playwright Version: "@playwright/test": "^1.15.2"
• Operating System: Catalina 10.15.7
• Node.js version: v14.16.0
• Browser: doesn't matter

Code Snippet

I've created repro here

Describe the bug

image comparator treats major changes in the same way as small ones,
so, minor changes in the picture must pass on lower threshold values comparing to major changed picture, but for now - both pass for the same 0.8

[yury-s]

This is how pixelmatch works. Both of the images have significant differences and they are flagged as such even with rather lax threshold 0.7 (note that by default Playwright uses more strict 0.2).