[stella] Z26 and Score Mode ball priority

[stella at casperkitty.com]

In Z26, in Score mode, the ball and both sprites have priority over the
playfield.  On a real 2600, on the left half of the playfield, the priority
sequence is:
  Playfield & Player/Missile 0
  Player/Missile 1
  Ball
  Background

On the right half of the playfield, the priority sequence is
  Player/Missile 0
  Playfield & Player/Missile 1
  Ball
  Background

The 2600's priority encoding is done based upon the color selection rather
than the "function" of a pixel.  The only exception is playfield priority;
that works by disabling the player0/player1 colors wherever the playfield
or ball appear.

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