### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

break;
		case COMPATIBILITY_TILE_MODE_ATLAS_TILE: {
			Array a;
			a.push_back(p_coords);
			a.push_back(p_flip_h);
			a.push_back(p_flip_v);
			a.push_back(p_transpose);
			return compatibility_tilemap_mapping[p_tile_id][a];
		}
		default:
			return cannot_convert_array;
			break;
	}
};
#endif 
bool TileSet::_set(const StringName &p_name, const Variant &p_value) {
	Vector<String> components = String(p_name).split("/", true, 2);
#ifndef DISABLE_DEPRECATED
	
	if (components.size() >= 1 && components[0].is_valid_int()) {
		int id = components[0].to_int();
		
		CompatibilityTileData *ctd;
		HashMap<int, CompatibilityTileData *>::Iterator E = compatibility_data.find(id);
		if (!E) {
			ctd = memnew(CompatibilityTileData);
			compatibility_data.insert(id, ctd);
		} else {
			ctd = E->value;
		}