buffer: Don't copy "memory" metas unconditionally
buffer: Don't copy "memory" metas unconditionally

Don't copy memory metas if we only copied part of the buffer, didn't
copy memories or merged memories. In all these cases the memory
structure has changed and the memory meta becomes meaningless.

https://bugzilla.gnome.org/show_bug.cgi?id=751712

jpxiong/gstreamer,centricular/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,krichter722/gstreamer,centricular/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer