Those are custom read/seeks that can transform data on the fly, for
games that use variations of regular FFmpeg codecs. When FFmpeg tries to
read N bytes, the code may read less/more but still fill the buffer with
N transformed bytes. Must handle "real" (data) and "virtual"
(transformed) offset/sizes.
Works with fake headers too, but "virtual" offset/size now include the
fake header, as I found hard to keep in mind when the header_size was
being added/substracted (could be clearer).
