app: add support for projectables with an arbitrary bounding box
In GimpProjectable, replace gimp_projectable_get_size(), which only returned a width and a height, with gimp_projectable_get_bounding_box(), which returns a full rectangle. This allows projectables to have an arbitrary bounding box, not limited to a (0, 0) top-left corner. Adapt GimpProjection, creating a buffer with corresponding extent to the projectable's bounding box. Adapt GimpImage and GimpGroupLayer.
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