Options/properties that are choices, and which include "yes" or "no"
values (or both) can now be read and written as MPV_FORMAT_FLAG.
For write access, rejecting flags in these cases was obnoxiously
unintuitive and inconvenient.
For read access, the value of this is less convincing, and actually it's
a major API change. At this point I probably have to admit that the
finer details of the client API are very unstable.