Make buffer structure sharable

Use stucture offsets instead of pointers so that we can share the buffer
structure between client and server.
We can then pass an fd wrapping the memory of the buffer to the client.
Add beginnings of a memory pool

spa/lib/meson.build

@@ -1,6 +1,7 @@
 spalib_sources = ['audio-raw.c',
                   'control.c',
                   'debug.c',
+                  'memory.c',
                   'props.c',
                   'ringbuffer.c',
                   'video-raw.c']