active_conn_proxy: Fix connection chains with changing param/ret types
active_conn_proxy: Fix connection chains with changing param/ret types

Previously, this code:

    pure::event_source<double> src;
    src >> [](double d) { return string{}; }  >> [](string) {} ..;

would result in a a chain of active_connection_proxies until something
with the signature void(double) was connected. This was caused by the
is_passive_sink_for check using active_connection_proxy::result_t, which
would (for the active_source_first policy) always return the result_t of
the active port.

By correcting the result_t of active_source_first, all the surrounding
logic works correctly again. This allows chains of lambdas that change
return types and parameter types to be formed (again).

CasparKielwein/flexcore,FlexCoreLib/flexcore