hooklib: Split out of util

This is distinct from capnhook's hooklib, although the intent is
the same. Hopefully these two libraries sharing a name won't get
too confusing...

hooklib/gfx.h

@@ -0,0 +1,4 @@
+#pragma once
+
+void gfx_hook_init(void);
+void gfx_set_windowed(void);