perf(图片加载): 优化图片加载逻辑，简化解码处理

移除冗余的解码检查逻辑，直接使用现代浏览器的异步解码特性
添加fetchPriority优化加载优先级，提升页面性能

js/shapes/utils.js

@@ -21,14 +21,9 @@
     const img = new Image();
     const record = { img: img, ready: false };
     window.YooneSnowImageCache[imageUrl] = record;
-    img.onload = function(){
-      var decoder = img.decode && typeof img.decode === 'function' ? img.decode() : null;
-      if (decoder && typeof decoder.then === 'function'){
-        decoder.then(function(){ record.ready = true; }).catch(function(){ record.ready = true; });
-      } else {
-        record.ready = true;
-      }
-    };
+    try { img.decoding = 'async'; } catch(e) {}
+    try { img.fetchPriority = 'low'; } catch(e) {}
+    img.onload = function(){ record.ready = true; };
     img.onerror = function(){
       // 加载失败 从缓存移除避免重复错误
       delete window.YooneSnowImageCache[imageUrl];