AF458
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Battle background movement of the "full screen" style (as opposed to 
mode-7-style shifting, which is controlled by AF908.)
Ten-byte entries.

[AAAA BBBB CCCC DDDD EEEE]


AA - Duration (measured in 1/60ths of a second?)
BB - Horizontal movement.
CC - Vertical movement.
DD - Horizontal acceleration.
EE - Vertical acceleration.


Horizontal:
  Positive left
  Negative right

Vertical:
  Positive up
  Negative down

.