libs: video: split allocation query caos and pad caps

libs: video: split allocation query caos and pad caps

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

This patch splits this logic distinction for videodecoder and videoencoder.

The user if needs a different allocation caps, should set the allocation_caps
in the GstVideoCodecState before calling negotiate() vmethod. Otherwise the
allocation_caps will be the same as the caps set in the src pad.

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