OMX's allow 3rds party to define extensions using their own enums
(like OMX_VIDEO_CODINGEXTTYPE) and to be used as the general
ones (like OMX_VIDEO_CODINGTYPE).
Properly cast those to fix -Wenum-conversion warnings from some
compilers such as clang.
https://bugzilla.gnome.org/show_bug.cgi?id=789057