app: add transform_valid member to GimpTransformTool
... which specifies whether the transform matrix is valid. Subclasses can then set this member to indicate that the current transformation is invalid (which can happen in the unified, perspective, and handle transform tools). When the transform is invalid, GimpTransformTool doesn't apply it upon committing, and returns an error instead. This commit doesn't set the transform_valid member in any of the subclasses, so there's no effective change. The next commit adds a GimpGenericTransformTool subclass, that will use the new member.
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