pointcloud manager/pointcloudviewerdx11: slicing issue and bounds not updated if set new point data

one report about, pointcloud manager sometimes failing on the slicing (and missing bounds if set new data at UpdatePointData())
> PointCloudManager.cs from line 126 Adding Mathf.Floor and Ceil to the min and max values. This is required as SX etc at line 254 are ints. BoxIndex was returning larger than array size.
> PointCloudViewerDS11.cs UpdatePointData() Need to find and set cloudBounds.
