A skipped frame is detected when the producer offers a frame while the
current pending frame has not been consumed.
However, the producer (in practice the decoder) is not interested in the
fact that a frame has been skipped, only the consumer (the renderer) is.
Therefore, expose the skipped count in consumer_take_frame() instead of
a flag in the producer_offer_frame().
This allows to manage the skipped and rendered frames count at the same
place, and remove fps_counter from decoder.
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