Entity co-ordinates should be in worldspace

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Currently, entities are placed relative to the screen. this makes things awkward in that moving backwards seems to be significantly slower than moving forwards.
it also makes it difficult to assign basic speeds to ships or powerups, as the level scrolling makes stationary objects seem to be "matching" with the player, while making objects stay put with the level requires making them move left-ward at a specific rate (which is not only annoying, but would cause issues if we ever decide to change the rate at which the level scrolls.)