create separate mailbox structures for draw and input messages, because our plan is to: 'handle input + drawing on a single thread'

shell/draw-mailbox.sml

@@ -0,0 +1 @@
+structure DrawMailbox = MakeMailbox(DrawMsg)

shell/input-mailbox.sml

@@ -0,0 +1 @@
+structure InputMailbox = MakeMailbox(InputMsg)

shell/make-mailbox.sml

@@ -0,0 +1,29 @@
+signature MAKE_MAILBOX =
+sig
+  type t
+end
+
+functor MakeMailbox(Fn: MAKE_MAILBOX) =
+struct
+  val messages: Fn.t vector ref = ref #[]
+
+  fun getMessagesAndClear () =
+    let
+      val () = MLton.Thread.atomicBegin ()
+      val msgs = !messages
+      val () = messages := #[]
+      val () = MLton.Thread.atomicEnd ()
+    in
+      msgs
+    end
+
+  fun append newMsg =
+    let
+      val () = MLton.Thread.atomicBegin ()
+      val msgs = !messages
+      val msgs = Vector.concat [msgs, #[newMsg]]
+      val () = messages := msgs
+    in
+      MLton.Thread.atomicEnd ()
+    end
+end

shf.mlb

@@ -66,6 +66,14 @@ in
   ffi/glfw-input.sml
 end
 
+ann
+  "allowVectorExps true"
+in
+  shell/make-mailbox.sml
+end
+shell/input-mailbox.sml
+shell/draw-mailbox.sml
+
 shell/exception-logger.sml
 shell/search-thread.sml
 shell/update-thread.sml