Bug 730071 - Selection bounds miscalculated for multiples of 128

gimp_gegl_mask_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".