A previous commit took care to make Decoder::position() not be updated

A previous commit took care to make Decoder::position() not be updated
if the data that was emitted from the decoder was not taken by the player.

This means that when the decoder moves into its end trim the position will
stay where it is (since the player does not take the data).

I can't see the point of doing this; the only use of Decoder::position()
is to decide what to pass() next (I think).

It is also inconvenient because we would like to check Decoder::position()
to decide whether to stop passing a decoder since it's in its end trim
(not doing this causes #1154).