vsgbuilder instancing example

[lufriem]

Based on the discussion in #935 I created a little (?) example, based on a stripped-down vsgbuilder code, demonstrating how to update instance properties (link to the branch).

Basically, this sample, if run without command line arguments, creates 3000 sphere instances (just like the vsgbuilder example), then updates the color of each sphere (one update per frame) until all spheres have the same color, then recalculates that color and does the cycle anew (so this demonstrates how to change the sphere instance property).

In addition, a single sphere will change its position; it will bounce randomly within the volume of created spheres.

The code's organized as follows

• there is a 'simulation update operation' struct, derived from vsg::Operation, which is run once per frame
  • this updates a 'simulation model' class instance (which computes the properties of the instances). Its update rate is hence coupled to the frame rate, which is probably not very life-like but easier to do
  • it then also applies a 'simulation visitor' struct to the scene which gets updates from the model and applies them to the scene

What the sample doesn't demonstrate

• billboarding (but that should be manageable, after all, the vsgbuilder example does it)
• if the scene has more than a single node, how to find the correct node (actually I'm not sure how to do that, I guess an auxiliary needs to be added? But how can this be done when using the Builder class? Something to figure out I guess..)

Well, it might be useful for something..