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.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: MIT
 /*
  * Compton - a compositor for X11
  *