lib/lp-scene.c: Deadlock on scene disposal

[rodrimc]

Disposing the scene while there is a media playing may lead to deadlock. The function scene_step_unlocked() blocks indefinitely after calling g_main_context_iteration().

Here is the relevant part of the backtrace.
#0 0x00007ffff63f03ed in poll () from /lib64/libc.so.6
#1 0x00007ffff76aacb6 in g_main_context_poll (priority=, n_fds=2, fds=0x7fffac0010c0, timeout=, context=0x81cdb0) at gmain.c:4135
#2 g_main_context_iterate (context=context@entry=0x81cdb0, block=block@entry=1, dispatch=dispatch@entry=1, self=) at gmain.c:3835
#3 0x00007ffff76aadbc in g_main_context_iteration (context=0x81cdb0, may_block=may_block@entry=1) at gmain.c:3901
#4 0x00007ffff7bce734 in scene_step_unlocked (scene=scene@entry=0x6b40b0, block=block@entry=1) at lp-scene.c:438
#5 0x00007ffff7bd009f in _lp_scene_step (scene=0x6b40b0, block=1) at lp-scene.c:1321
#6 0x00007ffff7bcd875 in lp_media_dispose (object=0x892000) at lp-media.c:1226
#7 0x00007ffff7983ea6 in g_object_unref (_object=0x892000) at gobject.c:3146
#8 0x00007ffff7bce4b3 in scene_stop_unlocked (scene=scene@entry=0x6b40b0) at lp-scene.c:387
#9 0x00007ffff7bced30 in lp_scene_dispose (object=0x6b40b0) at lp-scene.c:991
#10 0x00007ffff7983ea6 in g_object_unref (_object=0x6b40b0) at gobject.c:3146

To reproduce the deadlock, run the program play ($PROJECT_DIR/src/play.c) and close the window while the media is still playing.