app: in GimpTransformGridTool, allow linking forward/backward transforms
Add a GimpTransformGridTool::matrix_to_info() virtual function, which should extract the tool-specific transformation parameters given a transformation matrix, and the old parameter set (which is needed in some tools, to derive the parameters that aren't encoded in the matrix, such as the pivot point). The transformation matrix can be any combination of matrices calculated by the tool, and their inverses. Subclasses should only implement this function if every such matrix can be mapped back to transformation parameters. This is currently the case for all the transform-grid tools, except for the shear tool (since it only supports shearing along one of the horizontal or the vertical directions, however, the combined matrix may require shearing in both directions). When a transform-grid tool implements this function, show a chain- button between the two transform-direction radio-buttons in the tool options. When the chain-button is linked, whenever the transform corresponding to the active direction is modified, adjust the transform corresponding to the non-active direction such that the overall transform remains the same. One notable workflow that this enables is transforming a layer while adjusting a different area than its boundary, by first defining the area while the transform-directions are linked, and then transforming the area while the transform-directions are unlinked.
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