memory: Deprecate GST_MEMORY_FLAG_NO_SHARE
This flag always causes problems as it prevents subbuffering, instead one should create a custom GstAllocator to pool the GstMemory objects and not rely on the lifetime of the GstBuffer object they were originally attached to.
There is still code in the XImage src/sink elements as well as kmssink that use no-share, but we should document it so others don't use it in the future.
Originally proposed in. https://bugzilla.gnome.org/show_bug.cgi?id=757254