[jpegio] Use chunked reading if CFile::Open doesn't set CFile::GetFileSize

This solves the problem mentioned here:

http://forum.xbmc.org/showthread.php?tid=145656

in that some webservers don't set the filesize, or the file isn't seekable (with unknown filesize).

This fixes it by reading chunked.

Alternatives might be to have a general routine available to do this stuff such as ```CFile::ReadFile(std::vector<uint8_t> &foo)``` or some such.

Could also be done a little clearer perhaps by separating the chunked and non-chunked reads (the non-chunked read is a single Read() call like it was before).

@elupus, @theuni to review.