gegl_buffer_set, this allows transparent use of the buffers for writes
since only one write will happen at a time. (At the moment the lock
is implemented in the files header, and a GeglBuffer might get stuck
in a locked state on disk if gegl is aborted when it holds the lock.)
* gegl/buffer/gegl-buffer-access.c: (gegl_buffer_set): lock buffer
on write if the buffer is shared.
* gegl/buffer/gegl-buffer-index.h: fixed flags for header.
* gegl/buffer/gegl-buffer-private.h:
* gegl/buffer/gegl-buffer.c:
(gegl_buffer_is_shared): new function reports if the buffer is shared
(gegl_buffer_try_lock): tries to lock a shared buffer, if failed try
again a bit later.
(gegl_buffer_unlock): unlock a locked buffer, allows other clients to
use the buffer.
* gegl/buffer/gegl-tile-backend-file.[ch]: added _try_lock() _unlock()
* gegl/buffer/gegl-tile-backend.[ch]: added shared flag to backend.

svn path=/trunk/; revision=2258