Edges matter, part 2.

Affected modes: lighten, hard-light, overlay (== hard-light).
This fixes a couple places where I used < when I should have used <=,
or swapped the logic as I've done here.

Caught by layout tests; our tests should be unchanged.
https://storage.googleapis.com/chromium-layout-test-archives/linux_blink_rel/68935/layout-test-results/css3/blending/background-blend-mode-crossfade-image-gradient-diffs.html

BUG=skia:

Review URL: https://codereview.chromium.org/1217013003
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