Avoid loading OGG segments into memory

Avoid loading OGG segments into memory

Uses the virtual IO interface for sndfile providing pread() as the
mechanism for loading data from the file handle. pread() makes it
possible to read bytes from the file without impacting the offset
which would be altered with normal fread()/fseek() or
dup()+fread()/fseek() calls.

By avoiding changing the shared offset values, pread() makes it
possible to avoid multi-threaded data races while avoiding the
modest memory usage of loading a single audio track to memory.