... 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.