gstbuffer: _get_merged_memory: support memories that happen to be consecutive

Submitted by Ilya Konstantinov

Sometimes GstMemory objects might not be in a parent-child hierarchy which allows them to know that they span a single consecutive memory area.

However, when mapped, they end up being perfectly consecutive. For example, this can happen with GstCoreVideoMemory on Apple platforms.

Wouldn't it be nice if _get_merged_memory will recognize such cases and avoid allocating and memcpy'ing a new buffer in such cases?

Does this make sense?