[Feature Request] Use a generic TableProvider instead of directly depending on FontRef

[2A5F]

This will allow the use of other data sources, such as dwrite.

[LGFae]

I was looking into implementing a renderer where we only keep the information necessary for shaping in memory, and read and discard the glyph outline/bitmap information (since we cache it after the first time). This would really improve memory efficiency in my applications.

But that's only possible if I can use my own implementation of a TableProvider, instead of FontRef.