Spacescape

The stars no longer cluster at the right hand side after an abnormally
large delta
Flasher

Disable GL_BLEND once finished to avoid making this calculation when
not needed.
Spacescape, GameScreen, Repair, DrawShield

The Spacescape class is now the single actor that will be drawn by any
module wishing to show a view of the ship.

It should be shown/hidden in the activate/deactivate methods if
required (see Repair or DrawShield for examples).

Its zoom-level can be specified through the instance method
Spacescape.setZoom(…).

Refactor Repair and DrawShield modules to use this single spacescape
instead of creating one themselves.