mplayer: properly handle framestep when showing last video frame

When doing a framestep while there is no more video, nothing happened,
and audio continued to play. When advancing to the next file, the player
was paused. Fix it so that it always pauses (except on very low frame
rate video, which is yet another corner case).

We also change the meaning of framestepping a bit: in audio only mode,
framstepping unpauses for a single playloop iteration. This is probably
not useful at all, but makes the code a bit more simpler/uniform.

Just like the previous commit, this matters most for audio files with
cover art, for which this special case is the normal case.

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