[vtkusers] Rendering XYZRGBA segfaults randomly while XYZ doesn't in qvtkwidget subclass

[Oon-Ee Ng]

Additional point to note from more testing today:-

1. Adding a half second delay between calls to showPointCloud(...)
seems to alleviate the crashing as well as the artifacts.

2. Adding a mutex so data cannot be double-accessed in
VTKPointCloudWidget does NOT alleviate the crashing or artifacts, but
seems to help a bit (higher chance of running for more than 1 or 2
frames).

3. Removing NAN values from the point cloud does not seem to help appreciably