Fix in PCLVisualizer

Hi,
I tested the 'openni_fast_mesh' demo and got a warning of the visualisation modul that the polygon mesh named 'surface' is already existing. 

After debugging I found the reason in 'PCLVisualizer::updatePolygonMesh'. commit #686 replaced 'SetStrips()' by 'SetPolys()' in 'addPolygonMesh'. But then there must be also a replacement of 'GetStrips()' by 'GetPolys()' in 'updatePolygonMesh'.