https://github.com/OpenMW/openmw/commit/61cb5b4da638d520c41a9d99b5850384b2e22679

Index: extern/osg-ffmpeg-videoplayer/videostate.cpp
--- extern/osg-ffmpeg-videoplayer/videostate.cpp.orig
+++ extern/osg-ffmpeg-videoplayer/videostate.cpp
@@ -243,7 +243,11 @@ int VideoState::istream_read(void *user_data, uint8_t 
     }
 }
 
+#if FFMPEG_CONST_WRITEPACKET
+int VideoState::istream_write(void *, const unsigned char *, int)
+#else
 int VideoState::istream_write(void *, uint8_t *, int)
+#endif
 {
     throw std::runtime_error("can't write to read-only stream");
 }
