### 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)

for (KeyValue<Vector2i, TileAlternativesData> &E_alternatives : tiles) {
		for (KeyValue<int, TileData *> &E_tile_data : E_alternatives.value.alternatives) {
			memdelete(E_tile_data.value);
		}
	}
}
TileData *TileSetAtlasSource::_get_atlas_tile_data(Vector2i p_atlas_coords, int p_alternative_tile) {
	ERR_FAIL_COND_V_MSG(!tiles.has(p_atlas_coords), nullptr, vformat("TileSetAtlasSource has no tile at %s.", String(p_atlas_coords)));
	p_alternative_tile = alternative_no_transform(p_alternative_tile);
	ERR_FAIL_COND_V_MSG(!tiles[p_atlas_coords].alternatives.has(p_alternative_tile), nullptr, vformat("TileSetAtlasSource has no alternative with id %d for tile coords %s.", p_alternative_tile, String(p_atlas_coords)));
	return tiles[p_atlas_coords].alternatives[p_alternative_tile];
}
const TileData *TileSetAtlasSource::_get_atlas_tile_data(Vector2i p_atlas_coords, int p_alternative_tile) const {
	ERR_FAIL_COND_V_MSG(!tiles.has(p_atlas_coords), nullptr, vformat("TileSetAtlasSource has no tile at %s.", String(p_atlas_coords)));
	ERR_FAIL_COND_V_MSG(!tiles[p_atlas_coords].alternatives.has(p_alternative_tile), nullptr, vformat("TileSetAtlasSource has no alternative with id %d for tile coords %s.", p_alternative_tile, String(p_atlas_coords)));
	return tiles[p_atlas_coords].alternatives[p_alternative_tile];
}
void TileSetAtlasSource::_compute_next_alternative_id(const Vector2i p_atlas_coords) {