directwrite: split out the inner loop of scan_fonts as a separate function

This has the side effect that the ASS_FontProviderMetaData instance is now
cleared for every font rather than only once at the start of the search,
which fixes some use-after-free scenarios and prevents the creation of
chimeric fonts using names left over from other fonts processed earlier.

This lays the groundwork for further code simplification and error handling
improvements within this function, which will come in a separate commit.

This commit is transparent to `git blame -w` except for return statements.

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