Add gimp_item_scale_by_factors_with_origin(), which is an extension
of gimp_item_scale_by_factors(), taking the input/output points of
origin for the transformation (both of which are (0, 0) in the case
of gimp_item_scale_by_factors()).  Implement
gimp_item_scale_by_factors() in terms of the new function, and Use
the new function when scaling group layers, instead of manually
calculating the children boundaries, so that the behavior is
uniform across whole-image scaling and group-layer scaling.

The new function rounds all four edges of the boundary to the
image-global pixel grid, instead of only rounding the top/left
edges to the global grid, and the bottom/right edges to the item-
local grid.  This preserves layer-adjacency when scaling.