There are gstmemory's available that operate in two memory domains
and need to ensure consistent access between these domains.

Imagine a scenario where e.g. the GLMemory is mapped twice in both
the GPU and the CPU domain.  On unmap or a subsequent map, it would
like to ensure that the most recent data is available in the memory
domain requested.  Either by flushing the writes and/or initiating a
DMA transfer.  Without knowing which domain is being unmapped, the
memory does not know where the most recent data is to transfer to
the other memory domain.

Note: this still does not allow downgrading a memory map.

https://bugzilla.gnome.org/show_bug.cgi?id=750319