The current min/max zoom levels are supposed to be: 30% and 500%.
Before, due to floating point error accumulation in incremental addition
of zoom-step into zoom-level, one extra zoom step would get allowed,
enabling user to zoom 20%-to-510%.
Now, using rounding, the intermediate zoom-level values should be as
close to the theoretical value as FP32 can represent. E.g. zoom-level of
70% (theoretical multiplier 0.7) is 0.69... .
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