[Bug] StreamReader_istream fails on SMB/CIFS filesystems due to `tellg()` returning -1

Environment:
- OS: Linux
- Filesystem: SMB/CIFS (Windows Server share)
- libheif version: 1.19.8

Problem:
`tellg()` returns `-1` on SMB mounted files, causing "No 'ftyp' box" error

Root cause:
`StreamReader_istream` constructor in `bitstream.cc` uses `seekg()`/`tellg()`
which is not reliable on CIFS filesystems

Suggested fix:
Use `stat()` or pass file size from `HeifFile::read_from_file()`