Expose outline format in TableProvider

[dfrg]

Adds a new OutlineFormat enum and outline_format method to TableProvider. Impl in FontRef bases this on sfntVersion.

While this was generally accessible from FontRef, the skrifa scaler only accesses fonts through TableProvider and this info will be necessary for passing some of the Unicode text rendering tests (those that include both glyf and CFF tables and expect the choice to be made based on sfntVersion) in the future.

Also fixes #477