Improve IBO construction performances in vtkOpenGLIndexBufferObject

For the common cases of triangles-only polydata, the IBO construction time as been reduced by about 25%. It is even more faster if the polydata cell arrays use 32bits connectivity IDs. This can be forced using the VTK_USE_64BIT_IDS=OFF when building, or using vtkCellArray::SetDefaultStorageIs64Bit(false); or polydata->GetPolys()->ConvertTo32BitStorage().