15998: Fix mismatched new/free in vtkOpenGLImageSliceMapper.
By default, the SetArray() method uses free() rather than delete [] to release the memory, which in this case is allocated via new [] in the MakeTextureData() method.
By default, the SetArray() method uses free() rather than delete [] to release the memory, which in this case is allocated via new [] in the MakeTextureData() method.