CvPlot::setMaxVisibilityRangeCache should set a realistic max

[Nightinggale]

CvPlot::setMaxVisibilityRangeCache() is called at startup and it determines how far a unit can see in theory. This is then used in the code when scanning the map for units, which might see a certain plot. The smaller this value, the faster the scan.

The issue

The current code loops all promotions and adds up all visibility range changes. This means if a (land) scout can get +2 and a ship an get +2, the code says +4 even though no unit can get more than +2. This can easily result in a way too high MaxVisibilityRange.

Another issue is that it adds all of the values. If somebody adds a tradeoff promotion or a curse giving -1, then the number can become too low and that can then cause bugs.

Proposed solution

Make an EnumMap of UnitCombat, loop all promotion and calculate max viewing distance for each. EnumMap::getMax() then provides the MaxVisibilityRange.

Should only work on promotions providing > 0 visibility change.

Consider if other sources can affect this other than promotions.

[Nightinggale]

A bit of debugging reveals the game assumes units can view up to 10 plots, meaning if something updates on a plot, which affects visibility (like moving weather), it updates visibility for units in an area of 21x21 plots, which is 441 plots.

Perhaps the solution is more simple than it appears. When starting a game (including loading), CvGame sets this based on GC.getPLOT_VISIBILITY_RANGE() while pretending no unit can view longer than that. When CvUnit sets its own cache for how long it can see, it tells that to a static CvPlot function, which then increases if needed.

This means it will only check according to the unit with the longest viewing range, which was ever in the game. Loading a game can even reduce this if the longest viewing one died.

Reported viewing range	Plots to check	Reduction in number of plots to check
1	25	94.33%
2	49	88.89%
3	81	81.63%

Note: checked distance is one more than reported because of tall terrain being visible at one plot further away, like ships detect land at +1 range.

In other words a significant difference in the number of plots to check when using this approach. As the limit increases with units, the risk of it going out of sync seems as close to non-existing as possible.

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A critical function to consider for this to work is CvUnit::visibilityRange(). It allows improvements to increase visibility range, hence max improvement visibility boost should be considered. I think it's 0 right now, but even then it should be included or it would be a bug waiting to happen.