playtree: --shuffle shouldn't make playlist navigation useless

When --shuffle was used, the pt_step -1 jumped to the next file, instead
of the previously played file. This was because the playtree entries were
never actually shuffled, but instead a random unplayed file was picked.
Fix this by actually shuffling the playtree in advance. I couldn't see any
clear location where exactly this should happen, so it's done when a
playtree iterator is created.

Not removing the old code, since new playtree entries could be added while
an iterator is active.

0 files changed, 0 insertions, 0 deletions