More reliable ear test and point in triangle arithmetic

[mourner]

• Closes Errors when a hole is touching an outer ring #39 in theory (need to verify on all cases) for realz.
• Improves triangulation quality on 2 of the big fixtures.
• Generally makes all point in triangle tests more numerically robust and reliable. May help with Port bugfixes from JS earcut.hpp#6 (comment).
• Makes the code simpler and less verbose.
• Degrades performance a bit (about 6%), but I think that's a fair price to pay for more robustness.

cc @jfirebaugh @springmeyer @yhahn