Allow Shield2Gold_Factor to depend on the unit

[lmoureaux]

Is your feature request related to a problem? Please describe.
Corbeau tried to use that in Sim but it doesn't work.

Describe the solution you'd like

The effect is Shield2Gold_Factor. The following is written by someone who has only a rough understanding of how the game works internally (that'd be me): The only place where I could find it used is when calculating unit upkeep in common/unittype.cpp, line 145. You could probably change the call from get_player_bonus to get_unittype_bonus. I don't know if it would have any unintended consequences or whether it's even possible. Seems like you'd have to retrieve the tile the unit is on, since that's not used in that piece of code at all.

My notes:

• Looks like the AI doesn't know about Shield2Gold_Factor anyway
• Getting the tile the unit is on is easy, need to be careful that it could be nullptr

Describe alternatives you've considered

Doing nothing.

Additional context

Discussion on Discord

[lmoureaux]

More Discord discussion

The issue is the effect is evaluated using get_player_bonus, while it should really be get_unit_bonus (or get_unittype_bonus if changes for a unit bonus are too deep):

freeciv21/common/unittype.cpp

Line 141 in c028f42

gold_upkeep_factor = get_player_bonus(pplayer, EFT_SHIELD2GOLD_FACTOR);