Sort out license problems

IANAL, but I think I am allowed to add missing copyright notices for
someone else. And I did my best job using git history to figure out who
wrote which functions. So I hope everything is fine.

src/opengl.h

@@ -1,4 +1,4 @@
-#pragma once
+// SPDX-License-Identifier: MIT
 /*
  * Compton - a compositor for X11
  *
@@ -9,6 +9,8 @@
  *
  */
 
+#pragma once
+
 #include "common.h"
 
 #include <ctype.h>