KisAbstarctProjectionPlane makes the abstractions of a merge'able entity.
It regenerate() it when the entity is filthy, fetches need/change/accessRect()
when needed and apply() the entity to the global projection finally.

Now KisAsyncMerger and Kis*Walker use this interface only, instead of
old-fasioned direct methods of KisNode/KisLayer.

The general idea is:

KisLayer uses all its internal framework to generate a projection()
device. The user sets a layer style on a layer by setting a special
projectionPlane() interface, which wraps this generated prejection()
data into Layer Styles.

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