(This commit also modifies deps/media-playback)
Before, the media-playback library would detect whether something was
seekable by checking the filename for "://", which is unideal because
there are other cases where targets may not be seekable. So instead, an
explicit "seekable" property (off by default) is now in place in the
media source when not in "local file" mode. Seeking will only be
enabled if local file mode is on, or if "seekable" is explicitly checked
by the user.
Closesjp9000/obs-studio#1022
c9182a9878