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.