With APZ, we always layerize the first scrollable element of the page,
if the page itself is not scrollable. These additional layers can cause
fuzzy reftest failures in two ways: differences in blending, and by
disabling sub-pixel test anti-aliasing. The latter is only a problem
with unaccelerated drawing, because we don't support component alpha
layers with BasicLayers. (We also don't support them with
BasicCompositor, but "Reftest unaccelerated" only tests BasicLayers).
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