* app/undo.c: Fixed another use of array of gpointer in place of a
	proper struct, this time in undo_(push|pop)_channel_mod.

	* base/base-types.h
	* base/tile-manager.h
	* base/tile-manager-private.h
	* base/tile-manager.c: added PixelDataHandle as an abstraction
	on top of tiles.  PixelDataHandles either return a pointer into
	the tile data, or create a temporary buffer so the calling
	function can access data from disparate tiles using a single
	buffer.  This is a step in reducing the dependence of core
	image functions being aware of tiles as well as a step toward
	having a single abstraction for pixel data.

	* app/image_map.c: changed to use read_pixel_data_1
	* app/pixel-region.c: changed to use the read_pixel_data and
	write_pixel_data where practical.