Avoid closing stream by OpenFontReader.ReadPreview

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.