Bug 730071 - Selection bounds miscalculated for multiples of 128

gimp_channel_real_bounds(): when we succeed avoiding iterating a tile
by checking its upper-left and bottom-right, use the function's
internal meaning of x2, y2, which is the rightmost/bottommost selected
pixel, while the external meaning is the pixel right/below it.
Short: use "foo - 1" not "foo".