Rename Asteroid.prefab to Asteroid1.prefab
Add Asteroid2 to Scene and give it a rigidbody
Add Collider, Scripts, fx to Asteroid2, turn it into prefab

* Capsule Collider, not a trigger
* RandomRotator.cs, DestroyByContact.cs, Mover.cs (Same as Asteroid1)
* Turn it 180 degrees around Y-axis so it moves in the right direction
* Give it same speed and tumble as Asteroid1
* All explosion effect
Add Asteroid3 prefab, same treatment as 1 and 2
Spawn all asteroids with equal chance

* Speed in Mover scrips changed to negative to compensate for
  Asteroids2&3 facing the other way
* Correct the hierarchy of Asteroids2&3. Now props are child objects
  of a new GameObject with scripts, rigidbody, collider attached.
  This is just like in Asteroid1.
Add missing Asteroid models for asteroids 2&3
Reduce the size of the background texture

* Verified to take just over 1MB off the PC build size
* New image is scaled down to match the target resolution of 600x900
* The parts of the image off the camera are set to solid black.
  This is to keep the convenient overall size (easlier to scale and
  position) but have a smaller compressed size (solid colour compresses
  well).
  Unity automatically compresses all imported textures.
* PNG format chosen over others to keep the file size in the GIT repo
  to the minimum.
  Unity post-processes imported files anyway, so file type doesn't
  affect it.
Remove unnecessary Mesh Collider from Background
Add EnemyPreSpawnWait to fix early explosions
Improve+inc. Enemies' random move, inc. fire rate

* Fire rate was every 2s, now it's every 1s
* Random move was increased from 2 to 5
* Random move now only applied when no obstacle was detected