caching for "scanning a plane with a tile-compositing accessor"

[ptahmose]

c.f. #73 - for the use-case where a plane is swept with a ROI (in case of using an accessor which performs tile-composition), the performance (in case of CZIs containing compressed data) is expected to greatly benefit from caching the decompressed bitmaps.

Esp. this scan-pattern is of interest here:

An implementation has to allow for controlling the memory-usage of the cache, and it must need to deal with cache eviction. The versatility of libCZI-implementation should not be compromised.

[ptahmose]

As far as libCZI is concerned, I'd consider this task as done.