Avoid closing stream by OpenFontReader.ReadPreview

[acaly]

When using OpenFontReader to read a font collection, it would be really helpful to keep the stream open such that ReadPreview and Read can be called on the same stream instance. If the stream is seekable, this simplifies the logic a lot, because otherwise we either need to manually make a copy (probably to MemoryStream) or open the file again.

[prepare]

Thank you , I will check it again