path ops stream-lining

path ops stream-lining

The addT() function is a workhorse of pathops.
Make it simpler, removing branches and parameters.

Separate addOpp() into const and modify parts.

Add more debugging that asserts if the function
fails and the data is not extreme (e.g., fuzzer
generated).

TBR=reed@google.com
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Review-Url: https://codereview.chromium.org/2273293004