Fix Bug svg#1946

Optimize a large number of vectors sequentially instead of parallel to avoid the simultaneously opened file limit

lib/svgo/coa.js

@@ -303,20 +303,27 @@ function optimizeFolder(config, dir, output) {
 function processDirectory(config, dir, files, output) {
   // take only *.svg files, recursively if necessary
   var svgFilesDescriptions = getFilesDescriptions(config, dir, files, output);
-
-  return svgFilesDescriptions.length
-    ? Promise.all(
-        svgFilesDescriptions.map((fileDescription) =>
-          optimizeFile(
-            config,
-            fileDescription.inputPath,
-            fileDescription.outputPath,
-          ),
-        ),
-      )
-    : Promise.reject(
-        new Error(`No SVG files have been found in '${dir}' directory.`),
-      );
+
+  if ( svgFilesDescriptions.length == 0 ) {
+    return Promise.reject(new Error('No SVG files have been found in "' + dir + '" directory.'));
+  }
+
+  var semaphore = Promise.resolve([ ]);
+
+  svgFilesDescriptions.reduce(function(semaphore, fileDescription) {
+    return semaphore.then(function(results) {
+      return optimizeFile(
+        config,
+        fileDescription.inputPath,
+        fileDescription.outputPath,
+      ).then(function(result) {
+        results.push(result);
+        return results;        
+      });      
+    });
+  }, semaphore);
+
+  return semaphore;
 }
 
 /**