memfs: allow owning non-HEAP buffers with ALLOW_MEMORY_GROWTH

Memory growth may obsolete HEAP references, hence memfs disables 'canOwn' and forces copying.
memfs tests for this situation and suggests using --no-heap-copy.

However, the test is too strict: the same warning also appears with --no-heap-copy.

The test needs only prevent copying from HEAP, and accept other sources (e.g. xhr.response).

src/library_memfs.js

@@ -273,19 +273,22 @@ mergeInto(LibraryManager.library, {
       //         of bytes anymore since their contents now represent file data inside the filesystem.
       write: function(stream, buffer, offset, length, position, canOwn) {
 #if ALLOW_MEMORY_GROWTH
-        // If memory can grow, we don't want to hold on to references of
-        // the memory Buffer, as they may get invalidated. That means
-        // we need to do a copy here.
+        // If the buffer is located in main memory (HEAP), and if
+        // memory can grow, we can't hold on to references of the
+        // memory buffer, as they may get invalidated. That means we
+        // need to do copy its contents.
+        if (buffer === HEAP8.buffer || buffer.buffer === HEAP8.buffer) {
 #if ASSERTIONS
-        // FIXME: this is inefficient as the file packager may have
-        //        copied the data into memory already - we may want to
-        //        integrate more there and let the file packager loading
-        //        code be able to query if memory growth is on or off.
-        if (canOwn) {
-          warnOnce('file packager has copied file data into memory, but in memory growth we are forced to copy it again (see --no-heap-copy)');
-        }
+          // FIXME: this is inefficient as the file packager may have
+          //        copied the data into memory already - we may want to
+          //        integrate more there and let the file packager loading
+          //        code be able to query if memory growth is on or off.
+          if (canOwn) {
+            warnOnce('file packager has copied file data into memory, but in memory growth we are forced to copy it again (see --no-heap-copy)');
+          }
 #endif // ASSERTIONS
-        canOwn = false;
+          canOwn = false;
+        }
 #endif // ALLOW_MEMORY_GROWTH
 
         if (!length) return 0;