Add gimp_tile_handler_validate_buffer_copy(), which can be used
instead of gegl_buffer_copy(), to copy a (subregion of a) source
buffer to a destination buffer with a GimpTileHandlerValidate,
uninvalidating, and avoiding unnecessarily rendering, the
affected region.  Additionally, if the source buffer also uses a
GimpTileHandlerValidate, the relevant parts of the source buffer's
dirty region are copied to the destination's dirty region as well.