directwrite: fix font collections

directwrite: fix font collections

DirectWrite's FontFileStream does not actually use the data of a specific
font in a collection, which was an expectation of the existing code. It
simply returns a stream to the underlying file, collection or not. So we
need to get the index of the font. This needs to be done lazily as this
information is only available in a FontFace, which is expensive to
initialize.

Add a new optional font provider function for lazy initialization of the
index and use it. This is similar to the check_postscript callback.

Fixes libass#275.

v2: fix type of returned value.