More graceful handling of large files

* Use 64 bit integer for total point cloud size, and read this from the
  las extended number of points record.  The actual number of points
  loaded remains a size_t.

* Catch the std::bad_alloc which is generated when we try to allocate
  too much memory and issue an error message.  This isn't really a great
  way to handle it, but it's better than crashing.

* Beef up error reporting for files which fail to load
Remove obsolete shader LoD multiplier