Implements DemOS core system services.

System provides:

• interrupt backup / setup / restore system
  
• standardized VBL routine
  
• 2 ring buffers:
  
  • one for core system (never freed)
    
  • another for transient demo screens data
    
• a bi-threaded system, mixed between cooperative (main => idle thread) and preemptive (idle thread => main)
  
  • one main thread: task executed here should be sliced by frame / synchronized with VBL
    At the end of main thread, main loop yield to idle thread background task with a call to SYSswitchIdle
    
  • idle thread: executes a background task not synchronized with the VBL. The task is restored in its last state then it is interrupted by VBL rescheduling the main thread
    
  • some simple synchronization primitives