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)
Status Job ID Name Coverage
  Preparation
passed #65543
gst indent

00:00:27

passed #65542
manifest

00:00:19

 
  Build
passed #65544
build fedora x86_64

00:03:29

 
  Test
passed #65545
check fedora

00:15:53

 
  Full Builds
passed #65546
build android arm64 api28

00:02:48

passed #65547
build cerbero fedora x86_64

00:11:20