Even if the demuxer cache does not multiple ranges yet. This is to
reduce the pain should caching of multiple ranges ever be implemented.
Also change it from the sub properties stuff to return a mpv_node
directly, which is less roundabout. Sub-property access won't work
anymore, though.
Remove the seekable-start/-end fields as well, as they're redundant with
the ranges.
All this would normally be considered an API change, but since it's been
only a few days with no known users, change it immediately.
This adds some node.c helpers as well, as the code would be too damn
fugly otherwise.
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