Improved degenerate barycentric check

[IanLilleyT]

When the three points given to the barycentric function are colinear (which is not a valid triangle), the barycentric calculation will return a nan by doing 0/0. This new check safeguards against that happening, and ends up returning 1,0,0.

Also I replaced the old spec because it seemed to be doing the same thing as evaluates on the p0-p1 edge test and also altered the p0 var which could be used for future tests (but luckily wasn't).

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

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[lilleyse]

One potential issue is that the code will return (1,0,0) even if the point doesn't touch the degenerate triangle.

Another approach might be to throw an error if the triangle is degenerate, which could occur whenever it's zero area (not just collinear).

[cesium-concierge]

Thanks again for your contribution @IanLilleyT!

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[IanLilleyT]

Since barycentric coordinates are not well defined for degenerate triangles we decided to return undefined in those situations instead of returning (1,0,0)