This can be used in a generic way as common interface by all platforms
that, in one way or another, pass around physical memory addresses.
This is used by the gl lib and seems useful enough, so might just as
well move it next to the other allocators.
https://bugzilla.gnome.org/show_bug.cgi?id=779067
Make a base class that can help with allocating fd-backed memory.
Make dmabuf extend from the base class.
We can now make methods to check if memory has an fd and get the fd for
all the different types of fd-backed memory.
Make a separate file for the code to handle the fd backed memory.
This would make it possible later to add other allocators also using
fd backed memory.
Create new GstMemory and GstAllocator base on dmabuf.
Memory is not allocated/freed by userland but mapped/unmmaped
from a dmabuf file descriptor when requested.
This allocator is included in a new lib called libgstallocators
https://bugzilla.gnome.org/show_bug.cgi?id=693826
