Suggestion: normalize grid to a cell size

Hello,

Since kingpin operates not with `visibleMapRect` but with a larger `bigRect` - this leads to that when we do enough panning (`[self _mapWasPannedSignificantly]` becomes YES), we see how existing clusters (clusters that existed before significant pan and will exist after this pan without changing the contents of annotations they are holding) change their positions more or less noticeably, and I feel this behavior being "nervous" for a potential user of such map.

This normalization trick prevents visual movements of clusters when panning map view - it is working fine for me and I don't see any drawbacks which would prevent me from using it.

Thanks.