Code warning cleanup and add GPL license header to all CPP files

RenderingManager.cpp

@@ -1,4 +1,23 @@
-#include <cstring>
+/*
+ * This file is part of vimix - video live mixer
+ *
+ * **Copyright** (C) 2020-2021 Bruno Herbelin <bruno.herbelin@gmail.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+**/
+
+#include <cstring>
 #include <thread>
 #include <mutex>
 #include <chrono>
@@ -38,7 +57,7 @@
 // standalone image loader
 #include <stb_image.h>
 
-// vmix
+// vimix
 #include "defines.h"
 #include "Log.h"
 #include "Resource.h"
@@ -48,6 +67,7 @@
 #include "SystemToolkit.h"
 #include "GstToolkit.h"
 #include "UserInterfaceManager.h"
+
 #include "RenderingManager.h"
 
 #ifdef USE_GST_OPENGL_SYNC_HANDLER