add points_in_polygon back to kiva.agg

[aaronayres35]

[targeting maint/5.2]

closes #842

This was originally removed in #787, however we still have users of this code so we are re-adding it here to avoid breaks with the upcoming release.

[rahulporuri]

here's what comes up when i search for from kiva.agg import in the enthought org on GitHub - of which only two instances of from kiva.agg import ..., points_in_polygon, ... are relevant to us. Let's discuss with the relevant internal team and hopefully fix the usage internally instead of merging/releasing this PR