op_bitrate() return value changes with tags, goes off the scale with large pictures attached

[DEATH923]

op_bitrate() uses whole file size (or link size) for bitrate calculation, not subtracting headers/tags.

Reported bitrate depends on tags present in the file, especially if there are attached pictures.

Sample file showing completely wrong bitrate: https://foobar2000.org/temp/bigcover.opus

Proposed fix: #64