ENH: Update vtkTextSource to support vtkAlgorithm::DesiredOutputPrecision

Change-Id: I3ff28f8707d43a12e99681f2a7fa0a2f6e12a662
ENH: Update vtkEllipticalButtonSource to support DesiredOutputPrecision

Change-Id: I475fb27b5135e93d0d42157ae59ed6a77771ce74