allocators: Add dmabuf-based GstMemory and GstAllocator

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
8 files changed