Implement preliminary version of "reverse" feature

Not hooked to all RGBGFX flags yet, but good enough for most use cases
(and as a base for future development, should I need to `reset --hard`.)

TODOs marked appropriately.

include/gfx/reverse.hpp

@@ -0,0 +1,14 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2022, Eldred Habert and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RGBDS_GFX_REVERSE_HPP
+#define RGBDS_GFX_REVERSE_HPP
+
+void reverse();
+
+#endif /* RGBDS_GFX_REVERSE_HPP */