fix: account for scroll offsets in downscaled screenshots

[ddussi]

Fixes #2684

When a screenshot size limit causes downscaling, viewport and element screenshots on a scrolled page capture the wrong region because their clip coordinates omit the page scroll offset.

Summary

• Return document coordinates from getSourceBox() by reading scroll offsets alongside the existing dimensions and device pixel ratio. Add the main page offset to element bounding boxes; keep full-page boxes at the document origin.
• Add three browser regression tests for emulated viewports, native viewports, and element screenshots. They check captured pixels after horizontal and vertical scrolling and verify that the scroll position is preserved.

Testing

All three regressions fail against unmodified source and pass with the fix.

• npm run test -- --test-concurrency=2: 897 passed, 3 skipped, 0 failed.
• Screenshot tests: 23 pass, including against the bundled build.
• npm run check-format and npm run gen: pass; no generated-file changes.
• NODE_OPTIONS=--max_old_space_size=4096 npm run bundle and npm run test:notices:no-build: pass.

Separate clipped-capture problems with pinch zoom and RTL layouts also reproduce before this patch and are not addressed here.

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