Generic font subsetter API

[mattl...@live.com]

Hi folks, I am having an "issue" with large PDFs and I had an idea on how to maybe expose the subsetter API so that I can implement this for outside ski: 

https://bugs.chromium.org/p/skia/issues/detail?id=10491

This is my first "feature request" and if it seems to work, then I don't mind actually implementing it.