mplayer: fix seek display during seeking when playing ordered chapters

The seek bar appeared to be "stuck" to the start of the current chapter.
This is a regression from 630a2b1. This commit assumed that hrseek_pts
would always contain the hrseek target time (when hrseek_active==true).
But this is not always the case: when playing timeline stuff (e.g.
ordered chapters), hrseek framedropping is abused to handle an obscure
corner case, and then hrseek_pts contains something completely unrelated
to the current playback time. See the added comment in mplayer.c and
commit c1232c9.

Fix this by trying something else to get a correct time "during"
hr-seeks. mpctx->restart_playback looks ideal, because it's set while
audio is being synced / audio buffers being filled, so we know that the
audio time is probably bogus while it is set. Let's hope this is
correct.

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