ENH: Faster low-level implementation for TransformPoints.

The vtkTransform's TransformPoints() method iterated through the
points using GetPoint() and InsertNextPoint(), both of which require
virtual method calls to vtkPoints' internal vtkDataArray. This patch
eliminates all per-point virtual method calls by typcasting to float
or double before looping through the points.

Change-Id: Ieabef02cddb6a3290a31315c8d5baf268dacc079
Merge topic 'transform-points-efficiency' into master

3c1ef1a4 ENH: Faster low-level implementation for TransformPoints.