Adds an API that allows providing pluggin alternate Tile storage backends in
GeglBuffer, this allows plugging existing low-level tile-level access APIs i
backends for GeglBuffer, permitting a low-level of integration with external
systems wrapping a GIMP, Krita, Blender, open streetmap or other (potentiall
mip-mapped) tile backends allowing unified access through the GeglBuffer API

This system should ease both migating GIMPs codebase to be more GEGL focused
even in a transition period where GIMP is not fully migrated to use GEGL.