The ear clipping method used by polyfill_2d only excluded concave ears
which meant ears exactly co-linear edges created zero area triangles
even when convex ears are available.
While polyfill_2d prioritizes performance over *pretty* results,
there is no need to pick degenerate triangles with other candidates
are available. As noted in code-comments, callers that require higher
quality tessellation should use BLI_polyfill_beautify.
09eb4fe19a