add framebufferSize callback (FFI scaffolding and creating new variant of input_message type but don't react to this message in a meaningful way yet)

ffi/glfw-input.sml

@@ -11,9 +11,13 @@ struct
 
   val exportMouseClickCallback =
     _export "mltonMouseClickCallback" public : (int * int -> unit) -> unit;
-
   val setMouseClickCallback = _import "setMouseClickCallback" public reentrant : window -> unit;
 
+  val exportFramebufferSizeCallback =
+    _export "mltonFramebufferSizeCallback" public : (int * int -> unit) -> unit;
+  val setFramebufferSizeCallback = 
+    _import "setFramebufferSizeCallback" public reentrant : window -> unit;
+
   (* Constants for mouse input. *)
   val (MOUSE_PRESSED, _) =
     _symbol "MOUSE_PRESSED" public : ( unit -> int ) * ( int -> unit );