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

[jwiggins]

See #556 for more details. Basically we'll need to do a bit of work to supply FreeType on Windows at build time if we want to remove it from kiva.

[jwiggins]

#556 was slightly misleading. I've since learned that Windows text rendering works fine, but there are issues with font engine caching in AGG which need to be worked around to get a correct result with respect to affine transformations. Celiagg was able to implement it in a way which works. (but it causes issue #643)