Remove vendored FreeType from Kiva Agg backend

[jwiggins]

Kiva's Agg backend includes FreeType 2.3.12 as a vendored dependency. This version is ~11 years old at this point.

Given the issues we've already had with symbol clashes from the vendored Agg code, it's probably a good idea to link FreeType as a shared library. This is already being done by EDM for matplotlib, so we could even use the same version.

#74 might be related