Chapter-specific postcard texture replacement

### I am planning on implementing this myself

No

### Describe your request

Currently, nearly every overworld sprite can be replaced on a map-specific basis except for the postcard sprite, and it would be really nice to be able to change that sprite without changing it for every map.

I might be able to implement it myself, I'm not sure, but I would just need some pointing in the right direction of where these texture replacements are handled.

### Additional context

_No response_