libs: audio: split allocation query caps and pad caps

libs: audio: split allocation query caps and pad caps

Since the allocation query caps contains memory size and the pad's caps
contains the display size, an audio encoder or decoder might need to allocate
a different buffer size than the size negotiated in the caps.

This patch splits this logic distinction for audiodecoder and audioencoder.

Thus the user, if needs a different allocation caps, should set it through
gst_audio_{encoder,decoder}_set_allocation_cap() before calling the negotiate()
vmethod. Otherwise the allocation_caps will be the same as the caps in the
src pad.

https://bugzilla.gnome.org/show_bug.cgi?id=764421