Always map MPV_FORMAT_STRING to setting property value directly through
M_PROPERTY_SET_STRING, instead of trying to go through
M_PROPERTY_SET_NODE.
This treats a direct MPV_FORMAT_STRING query differently from a
MPV_FORMAT_STRING wrapped in a mpv_node. This was already the case in
mpv_get_property(). The reason for all this is that mpv_node is supposed
to be the exact type, while a direct MPV_FORMAT_STRING goes through all
possible conversions.
Not sure if these semantics are good.