Fix thread safety issue in vtkUnstructuredGrid::GetCell

The vtkUnstructuredGrid::GetCell method that takes a vtkGenericCell is
supposed to be thread safe. However, there was an issue where GetCell
called a method that was unsafe under certain conditions.

The problem was thta GetCell was calling the version of
vtkCellArray::GetCellAtId that returned the pointer to the internal
array of indices. That was thread safe if vtkCellArray happened to use a
simple array of type vtkIdType to store the indices. However, if the
array was of a different type, it copied the values to an array stored
in vtkCellArray and a pointer to that is returned. This is very much not
thread safe.

Fixed the problem by using a different version of GetCellAtId that takes
in the vtkIdList of the generic cell. Since the connectivity has to be
copied to that array anyway, there is no real performance penalty for
this.

(cherry picked from commit 04885b5b)