a bit of refactoring

fcore/app-with.sml

@@ -0,0 +1,15 @@
+structure AppWith =
+struct
+  open AppType
+
+  fun bufferAndSize (app: app_type, newBuffer, newWidth, newHeight) =
+    let
+      val {buffer = _, windowWidth = _, windowHeight = _, startLine} = app
+    in
+      { buffer = newBuffer
+      , windowWidth = newWidth
+      , windowHeight = newHeight
+      , startLine = startLine
+      }
+    end
+end