Log Message:
graphics/camlimages: make it build again

Hasn't in a long time, so no PKGREVISION bump (it's at 96!)

It is fairly likely that these changes and/or OCaml internal changes
regarding immutable strings have changed the behavior if you abuse
internal interfaces or try to share graphics memory with the library
and scribble on it. Don't do that.

There are probably more data copies as well (meaning "slow") but I've
tried to avoid that as much as possible.