videos only play sound but no image

After an upgrade from Ubuntu 16.04 to Ubuntu 18.04, videos in pdfpc do not show image any more, only sound. I have created a minimal test case here (please fetch both the PDF and the mp4):
- http://stephane.magnenat.net/~steph/bug-pdfpc/bug-pdfpc.pdf
- http://stephane.magnenat.net/~steph/bug-pdfpc/gtc-studentgames-2018.mp4
The sources being:
- http://stephane.magnenat.net/~steph/bug-pdfpc/bug-pdfpc.tex
- http://stephane.magnenat.net/~steph/bug-pdfpc/video-title.png

Videos in the PDF play properly with other viewers, such as Okular, and also directly with gtc-play, and even with a vaapi chain: `gst-launch-1.0 -v filesrc location=videos/gtc-studentgames-2018.mp4 ! qtdemux ! vaapidecodebin ! vaapisink`.

Here is the log of pdfpc when launched with `export GST_DEBUG=*:WARNING`:

0:00:00.465308376 7478 0x55bff7509890 WARN GST_ELEMENT_FACTORY gstelementfactory.c:456:gst_element_factory_make: no such element factory "gtksink"!

(pdfpc:7478): GStreamer-CRITICAL **: 17:44:38.690: gst_element_link_pads_full: assertion 'GST_IS_ELEMENT (dest)' failed

(pdfpc:7478): Gtk-CRITICAL **: 17:44:38.690: gtk_widget_add_events: assertion 'GTK_IS_WIDGET (widget)' failed

(pdfpc:7478): GLib-GObject-WARNING **: 17:44:38.690: invalid (NULL) pointer instance

(pdfpc:7478): GLib-GObject-CRITICAL **: 17:44:38.690: g_signal_connect_object: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed

(pdfpc:7478): GLib-GObject-WARNING **: 17:44:38.690: invalid (NULL) pointer instance

(pdfpc:7478): GLib-GObject-CRITICAL **: 17:44:38.690: g_signal_connect_object: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed

(pdfpc:7478): GLib-GObject-WARNING **: 17:44:38.690: invalid (NULL) pointer instance

(pdfpc:7478): GLib-GObject-CRITICAL **: 17:44:38.690: g_signal_connect_object: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed

(pdfpc:7478): GLib-GObject-CRITICAL **: 17:44:38.690: g_object_set: assertion 'G_IS_OBJECT (object)' failed

** (pdfpc:7478): CRITICAL **: 17:44:38.690: pdfpc_view_video_add_video: assertion 'video != NULL' failed
0:00:00.530371509 7478 0x55bff7509890 WARN basesrc gstbasesrc.c:3583:gst_base_src_start_complete:<source> pad not activated yet
0:00:00.530948710 7478 0x55bff7509890 WARN basesrc gstbasesrc.c:3583:gst_base_src_start_complete:<source> pad not activated yet
0:00:00.580826092 7478 0x7f4bec183e80 WARN qtdemux qtdemux_types.c:233:qtdemux_type_get: unknown QuickTime node type .TIM
0:00:00.580849209 7478 0x7f4bec183e80 WARN qtdemux qtdemux_types.c:233:qtdemux_type_get: unknown QuickTime node type .TSC
0:00:00.580864547 7478 0x7f4bec183e80 WARN qtdemux qtdemux_types.c:233:qtdemux_type_get: unknown QuickTime node type .TSZ
0:00:00.581237383 7478 0x7f4bec183e80 WARN qtdemux qtdemux.c:3031:qtdemux_parse_trex:<qtdemux0> failed to find fragment defaults for stream 1
0:00:00.581447728 7478 0x7f4bec183e80 WARN qtdemux qtdemux.c:3031:qtdemux_parse_trex:<qtdemux0> failed to find fragment defaults for stream 2
0:00:00.582434207 7478 0x7f4bec183e80 WARN basesrc gstbasesrc.c:2445:gst_base_src_update_length:<source> processing at or past EOS

(pdfpc:7478): Gtk-CRITICAL **: 17:44:39.323: gtk_widget_set_opacity: assertion 'GTK_IS_WIDGET (widget)' failed
Screensaver inhibited
0:00:01.382838111 7478 0x55bff7adcc00 ERROR vaapivideomemory gstvaapivideomemory.c:508:gst_vaapi_video_memory_map: unsupported map flags (0x10003)
0:00:01.382874539 7478 0x55bff7adcc00 ERROR vaapivideomemory gstvaapivideomemory.c:508:gst_vaapi_video_memory_map: unsupported map flags (0x10003)
0:00:01.382899758 7478 0x55bff7adcc00 ERROR default video-frame.c:169:gst_video_frame_map_id: failed to map buffer
0:00:01.382926876 7478 0x55bff7adcc00 WARN videofilter gstvideofilter.c:341:gst_video_filter_transform_ip:<cairooverlay0> warning: invalid video buffer received

(pdfpc:7478): Gtk-CRITICAL **: 17:44:40.075: gtk_widget_set_opacity: assertion 'GTK_IS_WIDGET (widget)' failed

(pdfpc:7478): Gtk-CRITICAL **: 17:44:42.869: gtk_widget_get_parent: assertion 'GTK_IS_WIDGET (widget)' failed

** (pdfpc:7478): CRITICAL **: 17:44:42.869: pdfpc_view_video_remove_video: assertion 'self != NULL' failed
Screensaver reactivated

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: pdf-presenter-console 4.1-2
ProcVersionSignature: Ubuntu 4.15.0-23.25-generic 4.15.18
Uname: Linux 4.15.0-23-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CurrentDesktop: KDE
Date: Thu Jul 12 17:46:10 2018
InstallationDate: Installed on 2015-05-13 (1156 days ago)
InstallationMedia: Ubuntu 14.04.2 LTS "Trusty Tahr" - Release amd64 (20150218.1)
SourcePackage: pdf-presenter-console
UpgradeStatus: Upgraded to bionic on 2018-07-06 (6 days ago)