command: minor simplification
command: change what the metadata property returns

Change the type of the property from a string list (alternating
key/value entries) to a map. Using the client API, this will return
MPV_FORMAT_NODE_MAP, while Lua mp.get_property_native returns a
dictionary-like table.
command: allow changing filters before video chain initialization

Apparently this is more intuitive.

Somewhat tricky, because of the odd state after loading a file but
before initializing the VO.