A non-trivial last row of a user matrix (or transform) (something other than
[0 0 0 1]) is used correctly by OpenGL, but the GetBounds calculation in
vtkActor (as well as vtkImageActor and vtkVolume) used
this->Transform->TransformPoint() to trasnform the bbox from the mapper.  As
this->Transform is a vtkLinearTransform, the final row of the matrix is ignored
within TransformPoint (actually vtkLinearTransform::InternalTransformPoint(),
which is what is ultimately called).  Instead, we now use
vtkMatrix4x4::MultiplyPoint() to do the transformation.

Note, this change also includes  "cleanup" of several lines before and after
the for loop where we trasnform the points (within GetBounds() fn), replacing
the, with just this->ComputeMatrix().  This is a subset of what was removed,
as it was being called by this->GetMatrix(), but also note that internally
vtkProp3D::ComputeMatrix() does a Push() and Pop() on this->Transform just
as was done in the code that was removed.  There is a potential issue (I'm
looking for an extra set of eyes, or many eyes, to scrutinize) with the
GetOrientation() call (in ComputeMatrix()), which occurs before Push() and
which might screw things up since it results in vtkTransform::InternalUpdate()
which might mess up the Concatentation if doTheLegacyHack == true.  Even if
it is a (potential) problem, moving Push() before GetOrientation() (in
ComputeMatrix()) should (I believe) rectify the potential for a problem,
though I did NOT make that change... yet (waiting for feedback)

Change-Id: I122940f1c2142d4c64502a8a54c44a4e18bdc124