fix crashes due to glib ignored EDEADLK.

Taking a readlock after a writelock in the same thread should result
in a deadlock, yet pthread on Linux returns EDEADLK and continues.
glib-2.42.0 ignores EDEADLK and assumes the lock was taken. Releasing
the lock later causes issues: "Calling g_rw_lock_writer_unlock() on a 
lock that is not held by the current thread leads to undefined behaviour."

The issue at hand: 
 AudioStreamView::redisplay_track()
 -> foreach_region()                       #<< WriteLock
 -> add_region_view()
 ...
 -> AudioRegionView::create_one_wave()
 -> RegionView::update_coverage_frames 
 -> Playlist::top_unmuted_region_at()      #<< ReadLock

All current users of Playlist::foreach_region() are in the GUI
and AFAICT read-only (display regions, update visuals)

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