remove these assertions as they're not necessary

src/lib.rs

@@ -37,8 +37,6 @@ pub fn add_tiles(game_data: String, image: String, prefix: String) -> String {
 
     let width  = image.width();
     let height = image.height();
-    assert_eq!(width  % 8, 0);
-    assert_eq!(height % 8, 0);
     let columns = (width  as f64 / 8.).floor() as u32;
     let rows    = (height as f64 / 8.).floor() as u32;