### 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)

ERR_FAIL_COND_MSG(!tiles.has(p_atlas_coords), vformat("TileSetAtlasSource has no tile at %s.", String(p_atlas_coords)));
	TileAlternativesData &tad = tiles[p_atlas_coords];
	
	Vector2i new_atlas_coords = (p_new_atlas_coords != INVALID_ATLAS_COORDS) ? p_new_atlas_coords : p_atlas_coords;
	Vector2i new_size = (p_new_size != Vector2i(-1, -1)) ? p_new_size : tad.size_in_atlas;
	if (new_atlas_coords == p_atlas_coords && new_size == tad.size_in_atlas) {
		return;
	}
	bool room_for_tile = has_room_for_tile(new_atlas_coords, new_size, tad.animation_columns, tad.animation_separation, tad.animation_frames_durations.size(), p_atlas_coords);
	ERR_FAIL_COND_MSG(!room_for_tile, vformat("Cannot move tile at position %s with size %s. Tile already present.", new_atlas_coords, new_size));
	_clear_coords_mapping_cache(p_atlas_coords);
	
	if (new_atlas_coords != p_atlas_coords) {
		tiles[new_atlas_coords] = tiles[p_atlas_coords];
		tiles.erase(p_atlas_coords);
		tiles_ids.erase(p_atlas_coords);
		tiles_ids.push_back(new_atlas_coords);
		tiles_ids.sort();
	}
	tiles[new_atlas_coords].size_in_atlas = new_size;
	_create_coords_mapping_cache(new_atlas_coords);
	_queue_update_padded_texture();
	emit_signal(SNAME("changed"));
}
int TileSetAtlasSource::create_alternative_tile(const Vector2i p_atlas_coords, int p_alternative_id_override) {