This has multiple advantages:
* The macro has multiple parameters that often have similar or identical
values, yet very different meanings (one is the name of the
OptionsContext member where the parsed options are stored, the other
the name of the variable into which the result is written); this
change makes each of these explicit.
* The macro returns on failure, which may cause leaks - this was the
reason for adding MATCH_PER_STREAM_OPT_CLEAN(), also ost_add()
currently leaks encoder_opts. The new function returns failure to its
caller, which decides how to deal with it. While that adds a lot of
error checks/forwards for now, those will be reduced in following
commits.
* new code is type- and const- correct
Invocations of MATCH_PER_STREAM_OPT() with other types will be converted
in following commits.
e218bde9f9