add callbacks for mouse movement and mouse clicking

imperative-shell/input-callbacks.sml

@@ -0,0 +1,17 @@
+structure InputCallbacks =
+struct
+  open CML
+  open Msg
+
+  fun mouseMoveCallback mailbox (x, y) =
+    Mailbox.send (mailbox, (MOUSE_MOVE {x = x, y = y}))
+
+  fun mouseClickCallback mailbox (button, action) =
+    if button = Input.LEFT_MOUSE_BUTTON () then
+      if action = Input.MOUSE_PRESSED () then
+        Mailbox.send (mailbox, MOUSE_LEFT_CLICK)
+      else
+        Mailbox.send (mailbox, MOUSE_LEFT_RELEASE)
+    else
+      ()
+end