Media Kit: use the B_WRITABLE flag in plugins

The ffmpeg plugin reports some formats as read-only. But the media kit
included them in the list of writable formats anyway.

src/kits/media/AddOnManager.cpp

@@ -519,9 +519,13 @@ AddOnManager::_RegisterWriter(WriterPlugin* writer, const entry_ref& ref)
 		return;
 	}
 	for (uint i = 0 ; i < count ; i++) {
+		media_file_format fileFormat = fileFormats[i];
+		// Ignore non-writable formats
+		if ((fileFormat.capabilities & media_file_format::B_WRITABLE) == 0)
+			continue;
+
 		// Generate a proper ID before inserting this format, this encodes
 		// the specific plugin in the media_file_format.
-		media_file_format fileFormat = fileFormats[i];
 		fileFormat.id.node = ref.directory;
 		fileFormat.id.device = ref.device;
 		fileFormat.id.internal_id = info.internalID;