This was a bizzare piece of API that I inherited from cairo.  It has
been wrong adding them to cairo in the first place.  Remove them before
someone uses them!

This is the common convention for language binding tools.

Unlike the rest of the memory-mode enum, this one didn't only
describe the access mode of the input memory region.  Remove it.

If someone wants to inhibit duplicating, they can lock the blob
and throw away the key.

Based on mailing list discussion with Carl Worth.

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.