app: calculate bounding box of group layers
In GimpGroupLayer, when recalculating the group's size as a result of a change to one of the child layers (now including in response to a child layer's GimpDrawable::bounding-box-changed signal), calculate the group's bounding box (the bounding box of all its child layers' bounding boxes) alongside its logical bounds. Like in GimpLayer, use the logical bounds as the bounding box if the group has a mask. This bounding box is passed to the group's projection, via GimpGroupLayer's GimpProjectable::get_bounding_box() implementation, resulting in a buffer whose extent is the same as the bounding box.
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