Controlling Agent Visibility

[Robadob]

E.g.

• Can we opt out of rendering agent's in a given state (Sorted by Visualiser: Allow agent states to be hidden. FLAMEGPU2#1041)
• Can we use an agent variable to toggle visible/hidden
  • Should it be int treated as bool, or float with a custom condition?
• Can we use an agent variable to scale alpha (No, blending would require back-to-front rendering, requiring sorting of all agents to be drawn, not feasible.).

Current workout to hide agents would be to pass an invalid scale value such as NaN,

Suggested by Jack A