Only emit triangles, regardless of GLU_TESS_EDGE_FLAG

Triangle strips and fans are generally not desirable (or at least not a win) on modern hardware, and libtess's output is just a decent-effort greedy-algorithm attempt at them anyway, with the possibility of many small, disconnected primitives, requiring primitive restarts or degenerate vertices between them.

This (in-progress) change makes it so libtess will always supply `GL_TRIANGLES` as a result. Previously you could trigger this mode by passing in an `GLU_TESS_EDGE_FLAG` callback. It is now the only mode. You can still pass in a `GLU_TESS_EDGE_FLAG` callback to get callbacks on whether an edge lies on the polygon boundary or not (see original README), but it is no longer necessary to get the `GL_TRIANGLES`-only behavior.

Technically libtess could already output a list of triangles for any input and you had to be prepared for it to do so, so this isn't a breaking change per se, but it is a change of output for the same input, so we'll probably increment the minor version number.