close#1453
Events were delivered to the Flutter event sink directly on the mpv event
thread while method replies were already marshaled; concurrent messenger
use from two threads intermittently crashed guests during the event burst
of an in-place media switch. Also joins the event thread before draining
platform tasks in the plugin destructor (reverse member destruction order
would otherwise let the event thread enqueue into a destroyed queue) and
coalesces the wakeup PostMessage behind a retry-safe flag.
edde746
0e3c592205