out when allocation can't find a free area of the requested size. When
offscreen pixmaps get used, the offscreen area's score is increased by
a constant value. Every certain number of increases, all offscreen area
scores get decreased by a fraction. When choosing a set of areas to
remove for a new allocation, the set of areas with the smallest total
score is chosen for removal. While this is not the smartest system, it
prevents things like always removing the first offscreen area in memory
(likely the most recent) to be kicked out when doing replacing.
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