gst_audio_buffer_reorder_channels() was always mapping the buffer read-write
regardless whether any reordering was needed. If the from and to channel order
is identical return immediately without remapping the buffer.
Add a small helper function gst_audio_channel_positions_equal() which is used
in both gst_audio_reorder_channels() and gst_audio_buffer_reorder_channels().
https://bugzilla.gnome.org/show_bug.cgi?id=773833
We currently don't log much about channel positions making debugging
harder as it should be. This is the first step in my attempt to improve
this.
https://bugzilla.gnome.org/show_bug.cgi?id=763985
Rename _get_default_mask() to _get_fallback_mask() to make it more
clear that the function only provides a fallback if nothing else can be
done. Also clarify this in the documentation.
API: gst_audio_channel_get_fallback_mask()
