Roll tweaks and Drone convoys

Tweaked some gameplay related to the roll move, and added a new enemy encounter kind: drone convoys

• Made roll move not lock player for as long;
• Added Drone convoy enemy encounter;
  • Spawns a convoy of drones from off screen, moving through a fixed path on screen, before leaving the screen;
  • Each drone fires a single shot exactly halfway through the path;
  • Killing the leading drone (highlighted in red) kills all drones in the convoy.