Add copyright notice to backend/**

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/gl/gl_common.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Yuxuan Shui <yshuiv7@gmail.com>
 #include <GL/gl.h>
 #include <GL/glext.h>
 #include <locale.h>

src/backend/gl/gl_common.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Yuxuan Shui <yshuiv7@gmail.com>
 #pragma once
 #include <stdbool.h>
 #include <string.h>

src/backend/gl/glx.c

@@ -5,6 +5,7 @@
  * Based on `xcompmgr` - Copyright (c) 2003, Keith Packard
  *
  * Copyright (c) 2011-2013, Christopher Jeffrey
+ * Copyright (c) 2019 Yuxuan Shui <yshuiv7@gmail.com>
  * See LICENSE-mit for more information.
  *
  */

src/backend/gl/glx.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Yuxuan Shui <yshuiv7@gmail.com>
 #pragma once
 #include <GL/glx.h>
 #include <X11/Xlib.h>