keys property
MPV_FORMAT_NODE_ARRAY: unused (typically NULL), access is not allowed
MPV_FORMAT_NODE_MAP:
keysN refers to key of the Nth key/value pair. If num > 0, keys0 to
keysnum-1 (inclusive) are valid. Otherwise, this can be NULL.
The keys are in random order. The only guarantee is that keysN belongs
to the value valuesN. NULL keys are not allowed.
Implementation
external ffi.Pointer<ffi.Pointer<ffi.Char>> keys;