webgl tests broken/incomplete

[jerch]

@Tyriar

With #5852 the following webgl addon tests are wonky or literally wrong (testing for DOM nodes?), thus currently deactivated:

• [Chromium] › test/playwright/SharedRendererTests.ts:172:7 › WebGL Renderer Integration Tests › background 0-15 invisible
• [Chromium] › test/playwright/SharedRendererTests.ts:338:7 › WebGL Renderer Integration Tests › background 16-255 invisible
• [Chromium] › test/playwright/SharedRendererTests.ts:718:7 › WebGL Renderer Integration Tests › background true color grey invisible
• [Chromium] › test/playwright/SharedRendererTests.ts:875:9 › WebGL Renderer Integration Tests › minimumContrastRatio › should enforce half the contrast for dim cells
• [Chromium] › test/playwright/SharedRendererTests.ts:1180:9 › WebGL Renderer Integration Tests › regression tests › Invisible text is visible in the DOM renderer #4758: multiple invisible text characters without SGR change should not be rendered
• all webgl tests on safari (kinda all broken in CI)
• all webgl tests on firefox (passes locally, but not in CI)

Also see #5852.