Required for "modified-since".

All DetachedSong instances should be converted to LightSong first,
which is a very cheap operation.

This is kind of a revert of commit
b2e3c075, which is not any longer
necessary since commit 0dd4b52b
removed the last call to `av_open_input_stream()`.

Instead of copying the offset from our `input`, copy the requested
offset to our `offset` attribute.

By the time Seek() finishes, our input's offset may have advanced
already, having read some more data, thus giving us a bogus offset.

This fixes spurious decoder failues (closes #320).

Closes #89

Will allow more complex fitler expression, such as negation (#89).

Was converted to reStructuredText / Sphinx in commit
8a689e68

Not yet wired to the protocol.

Consume the ConstBuffer instead of iterating it.  This will allow
adding more complex parsers.

Having this as one-byte integer actually adds overhead and saves no
memory due to padding.