Cocoa: Fix conversion between NSPoint and CGPoint

GLFW still supports versions of macOS before these became the same
underlying type.

Closes #1023.

README.md

@@ -354,6 +354,7 @@ skills.
  - Jay Weisskopf
  - Frank Wille
  - yuriks
+ - Ryogo Yoshimura
  - Santi Zupancic
  - Jonas Ådahl
  - Lasse Öörni

src/cocoa_window.m

@@ -1030,7 +1030,9 @@ static GLFWbool createNativeWindow(_GLFWwindow* window,
     else
     {
         [window->ns.object center];
-        _glfw.ns.cascadePoint = [window->ns.object cascadeTopLeftFromPoint:_glfw.ns.cascadePoint];
+        _glfw.ns.cascadePoint =
+            NSPointToCGPoint([window->ns.object cascadeTopLeftFromPoint:
+                              NSPointFromCGPoint(_glfw.ns.cascadePoint)]);
 
         if (wndconfig->resizable)
         {