Todo:
        * Sprite priorities aren't verified to be 100% accurate.
          (Addendum - all known issues seem to be correct - see Sprite Priority Notes below).
        * There may be some kind of fullscreen palette effect (controlled by bit 3 in priority
          word - used at end of each level, and on final boss).
        * A shadow effect (used in level 1) is not implemented.

    Sprite Priority Notes:
        * On the Imperial Science Museum level at the beginning, you fly behind a wall, but your
          shots go in front of it. This is verified to be correct behavior by Guru.
        * There's a level where the player passes through several columns in a building and the
          player goes behind every 2nd one. That is correct as well.
        * There is a fire hydrant with an odd-looking spray of water on various levels.
          If you drop the hydrant in front of an enemy (a tank, for example), the priorities are
          wrong. This is actual PCB behavior. Also, the strange-looking water spray is correct.

    Alpha Blend Note:
        * There are semi-transparent round spots around your plane while fighting the final boss.
          These are correct.
        * The final boss shoots a blue beam downwards during the battle. This should have alpha.
          It fades in to blue, then fades out to nothing again after a few seconds (Guru).
          (Potentially related to note above about bit 3 in priority word)