[feature][API] add SaveAsImage()

Plugins/Download.jslib

@@ -0,0 +1,24 @@
+mergeInto(LibraryManager.library, {
+    Download: function (str, fn) {
+        var msg = UTF8ToString(str);
+        var fname = UTF8ToString(fn);
+        function fixBinary(bin) {
+            var length = bin.length;
+            var buf = new ArrayBuffer(length);
+            var arr = new Uint8Array(buf);
+            for (var i = 0; i < length; i++) {
+                arr[i] = bin.charCodeAt(i);
+            }
+            return buf;
+        }
+        var binary = fixBinary(atob(msg));
+        var data = new Blob([binary]);
+        var link = document.createElement('a');
+        link.download = fname;
+        link.href = URL.createObjectURL(data);
+        document.body.appendChild(link);
+        link.click();
+        document.body.removeChild(link);
+    }
+});
+