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