Generic unit, building, wonder icons

[lmoureaux]

Is your feature request related to a problem? Please describe.

When a tileset doesn't have an icon for an in-game feature, it refuses to load in the best case or crashes the game in the worst case.

This issue is about the graphics themselves, code to load them should be in separate issues.

Describe the solution you'd like

Draw and add to the client a set of generic unit icons. They shouldn't be tied to any tileset.

Describe alternatives you've considered

• Requiring tilesets to provide a generic icon themselves
• Using a random icon from the tileset
• Generating a red square icon

[daavko]

Possibly relevant links to Discord:
https://discord.com/channels/378908274113904641/830976751143485521/1032380414746185738
https://discord.com/channels/378908274113904641/830976751143485521/1032744534335029419