JPEG loader cannot read size header from stream

QImageReader::size() does not work for streamed JPEG (eg. QNetworkReply).

The cause is that the plugin doesn't create and maintain a jpeg_decompress_struct, but needlessly creates and destroys it when it is asked for the size. Like all other formats, libjpeg is designed to allow just the header (of streamed data) to be read. See qpnghandler for example of doing it correctly.