add missing license comment

main.cpp

@@ -1,5 +1,20 @@
-// fractal - OpenCL-accelerated Mandelbrot renderer.
-// Written by Clyne Sullivan.
+/**
+ * happy-fractal - A study of efficient and precise fractal rendering.
+ * Copyright (C) 2022  Clyne Sullivan
+ *
+ * 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/>.
+ */
 
 // If defined, program auto-zooms and measures runtime.
 //#define BENCHMARK