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: 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.