make SpecialLineRec::addSegment assert more permissive to accomodate rounding error

make SpecialLineRec::addSegment assert more permissive to accomodate rounding error

Because the driving code that calls SpecialLineRec::addSegment uses double precision,
when converted back to single precision this may round such that d0 and d1 are the same.
This causes the assert to trigger. The simplest thing to do here is to modify the assert
to not be so picky and allow the rare/occasional equality.

Downstream Firefox bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1290628

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2209303004

Review-Url: https://codereview.chromium.org/2209303004