Don't discard unused vertices

[jfirebaugh]

Discarding unused vertices makes earcut.hpp output unworkable for mapbox-gl-native due to the following:

• Fill outlines use GL_LINES with indexed vertices
• This requires knowing which vertex indexes correspond to which polygon rings
• We want to share the same vertex indices with fill triangles
• Therefore the indexes used by earcut need to correspond to the input vertex order -- no removal of vertices unused by triangulation
• Once that's the case, then we are able to loop through the vertices in input order, add them to the buffer, and add the indices of ring vertices to the line elements buffer

The JS implementation does not remove unused vertices.

cc @mourner @kkaefer to double check my logic.

[mourner]

Yes, after I switched the JS implementation to indexed output, the output indices now reference the original input array. Unused vertices are a rare occurrence anyway, and reusing the input array makes performance better.

[jfirebaugh]

Fixed by #10.