app: in GimpTransformGridTool, allow simultaneous forward and backward transforms
In GimpTransformGridTool, allow performing simultaneous forward (normal) and backward (corrective) transforms, by having each transform direction operate on an independent set of parameters. In other words, whereas the transform-grid tools previously had a single transform, which could be applied either normally or correctively using the "direction" tool-option, they now have two independent transforms, one applied normally and the other applied correctively, which are toggled using the "direction" option. The overall transform is the combination of the backward transform, followed by the forward transform. Another way to think about it, is that the tool transforms a source shape into a destination shape. The source shape is defined by the backward transform, and the destination shape is defined by the forward transform. Wherewas previously only one of these shapes could be controlled (the other shape always being the item bounds), it's now possible to control both shapes in a single transform. The next commit will allow modifying both shapes simultaneously, making this even more useful. Note that since both transforms start off as the identity, using only one of the transform directions has the same behavior as before.
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