In fit-down mode only 1.0 rates are supported, and the element will fit
audio data in buffers to their advertised duration.
This is useful in speech synthesis cases, where elements such as
awspolly will generate audio data from text, and assign the duration of the
input text buffers to their output buffers
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8149>