app: reduce time complexity of layer-group translation
When translating a layer group, avoid separately updating the original area of the child layers before translating them (as per the fix to issue #3484), as this results in quadratic time complexity w.r.t. to the maximal subgroup nesting level. Instead, simply defer the updating of the group's offset until *after* translating the child layers, so that their original area isn't clipped by the parent, while their new area is still properly updated even if the parent's size changes (see comment in code).
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