Bug 1533859 [wpt PR 15677] - Fix Acid2 to take the screenshot after it loads, a=testonly

Bug 1533859 [wpt PR 15677] - Fix Acid2 to take the screenshot after it loads, a=testonly

Automatic update from web-platform-tests
Fix Acid2 to take the screenshot after loading & scrolling

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Add a reference for Acid2 that should pixel-for-pixel match

Per https://bugzilla.mozilla.org/show_bug.cgi?id=343583#c6, Hixie
never intended the reference.png to match exactly (e.g., the nose
especially is quite fragile with anti-aliasing differences).

This is based on the test itself, but by and large just uses block
layout with pretty simple border/backgrounds. The only complexity is
in the eyes (where layering is needed to get the background
interpolated as on the test) and the nose (which mostly matches the
test as browsers trivially differ around the aliasing of
border edges).

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wpt-commits: 8a28e1a47287265e54f3c28650f3201f6f3e7dff, 42330b2ea1aa9e8c23fb2fbcfcba9f8cabd9bb18
wpt-pr: 15677