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

[LittleNewton]

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()