Factor the logic for iterating over whole-tiles contained in a
given rect, and over the remaining "edge" rects, out of
gegl_buffer_clear(), and into a new gegl_buffer_foreach_tile()
internal function, which takes a pair of functions: one which
performs the per-tile action, and one which performs the per-rect
action, and calls them for each tile/edge-rect in the given region.

Modify gegl_buffer_clear() to use the new function.

Additionally, when voiding the tiles, call
gegl_tile_handler_cache_remove() directly, to avoid voiding the
tile pyramid at each base tile.  Instead, have
gegl_buffer_foreach_tile() void the entire region in a single step.