well as synchronize a buffer so that it can be resumed from swap.
* gegl/buffer/gegl-buffer-load.c:
(gegl_buffer_read_header): moved magic check to header read.
(gegl_buffer_open): replaced by GeglBuffer being backed by a
serialized GeglBuffer.
* gegl/buffer/gegl-buffer.c: (gegl_buffer_constructor),
(gegl_buffer_class_init), (gegl_buffer_new_from_path):
made it possible to create buffer using a path.
* gegl/buffer/gegl-tile-backend-file.c: (flush),
(gegl_tile_backend_file_constructor): allow opening already
written swap files.
* gegl/buffer/gegl-tile-backend.h: provide some private storage to
store a pointer to the header used to read in a buffer.

svn path=/trunk/; revision=2217