Add license boilerplate

Signed-off-by: David Oberhollenzer <goliath@infraroot.at>

LICENSE

@@ -0,0 +1,13 @@
+Copyright (c) 2020 David Oberhollenzer <goliath@infraroot.at>
+
+Permission to use, copy, modify, and/or distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

shader.c

@@ -1,3 +1,9 @@
+/* SPDX-License-Identifier: ISC */
+/*
+ * shader.c
+ *
+ * Copyright (C) 2022 David Oberhollenzer <goliath@infraroot.at>
+ */
 #include "shadermeh.h"
 
 /* stub vertex shader */

shadermeh.c

@@ -1,3 +1,9 @@
+/* SPDX-License-Identifier: ISC */
+/*
+ * shadermeh.c
+ *
+ * Copyright (C) 2022 David Oberhollenzer <goliath@infraroot.at>
+ */
 #include "shadermeh.h"
 
 static GLfloat vertex_buffer[] = {

shadermeh.h

@@ -1,3 +1,9 @@
+/* SPDX-License-Identifier: ISC */
+/*
+ * shadermeh.h
+ *
+ * Copyright (C) 2022 David Oberhollenzer <goliath@infraroot.at>
+ */
 #ifndef SHADERMEH_H
 #define SHADERMEH_H
 

window.c

@@ -1,3 +1,9 @@
+/* SPDX-License-Identifier: ISC */
+/*
+ * window.c
+ *
+ * Copyright (C) 2022 David Oberhollenzer <goliath@infraroot.at>
+ */
 #include "shadermeh.h"
 
 #ifndef GLX_ARB_create_context