Consider whether MaxTextureAtlasSize should be a springsetting

- If MaxTextureAtlasSize is set too low, the ceg loading fails somehow and all ceg images are black squares.
- As far as I can tell there is no reason for MaxTextureAtlasSize to be set higher than necessary.
- Games have poor control over Springsettings, especially ones that must be set upon loading.

This raises the question of whether MaxTextureAtlasSize should be a modrule, or even just detected by the engine automatically (if it is reasonably cheap to do while loading). ZK has pretty good control of springsettings, but any mod that wants more texture space has no hope. Any game that supports more than one lobby is going to have trouble using anything beyond the default.

This is more a future proofing ticket than anything urgent.

No files or tests are named. Start by tracing MaxTextureAtlasSize through the CEG loading path and comparing SpringSettings with modrules; done means the project has an agreed, validated way to avoid black CEG images while supporting games and mods with differing texture needs.

Written by the indexing model from the issue text.