Part 3 - creating first version of "game loop" code, moving snowballs on screen

• there are 32 snowballs travelling at screen at two different speeds and left or right direction
• there is one static player sprite standing at bottom
• first version of game loop added:
  • uploads all sprites from memory to HW sprite engine
  • modify sprite data in memory for next frame
  • loop infinitely all that stuff

The result does "work", except everything is sort of too fast...
... something to resolve in Part 4.