@param opaque A private pointer representing current protocol.
It must be a pointer to NULL on first iteration and will
be updated by successive calls to avio_enum_protocols.
@param output If set to 1, iterate over output protocols,
otherwise over input protocols.
@return A static string containing the name of current protocol or NULL
Iterate through names of available protocols.
@param opaque A private pointer representing current protocol. It must be a pointer to NULL on first iteration and will be updated by successive calls to avio_enum_protocols. @param output If set to 1, iterate over output protocols, otherwise over input protocols.
@return A static string containing the name of current protocol or NULL