Add c3.555 loading in editor for game asset access

[dvincent56]

Engine: auxiliary main file in editor mode
To make Native_Meeting_Hut_Central_01 (which composites from c3.555 group 159) and the field variant sprites render correctly in the editor, the engine now loads c3.555 alongside c3map.555 as a secondary atlas (ATLAS_MAIN_AUX). XML assets opt into it with:
<layer source="aux" group="159" image="112"/>

Implementation mirrors the existing load_external_fonts pattern: separate storage (aux_data), own atlas, lookup via image_group_aux / image_get_aux, and external image handling for game assets that aren't atlas-packed. Game mode is unaffected — aux is only prepared when is_editor=1.

[crudelios]

If you don't mind, this PR actually is giving me the push to load all the original images at once to allow seamless climate/ editor transition (and have all images available at the same time).

I already have the arrays for the unique image ids per climate/editor, so it's just a matter of implementing it.

All in all, instead of loading 9000 images total, we'll be loading a little more than 10000, so I don't even know why the game originally uses three different files for the climates.