Allow having no visualization

Also allow visualizing each glyph as it's created, though there's no
code for pausing long enough to see the glyphs. The focus is producing
the TTF quickly, not producing pretty pictures.
Do not copy anchor points (for now)

Anchor points turn out to be more complex to copy: they rely on lookup
subtables, which are hard to copy. We'll be refactoring this code to
restore the ability to copy anchor points soon, but for now we'll just
disable the feature.
Be more flexible in looking for python-poly2tri

Now the p2t.so file can be searched for in the current directory, any
higher-level directory, or a python-poly2tri directory attached to any
of the above. This will make the software a little easier to install.
Add URL for python-poly2tri library