audio-converter: add API to determine passthrough mode

audioconvert's passthrough status can no longer be determined
strictly from input / output caps equality, as a mix-matrix can
now be specified.

We now call gst_base_transform_set_passthrough dynamically, based
on the return from the new gst_audio_converter_is_passthrough()
API, which takes the mix matrix into account.
6 jobs for audioconvert_no_passthrough_on_same_caps in 34 minutes and 18 seconds (queued for 45 minutes and 32 seconds)