Cocoa: Fix call to NSWindow from non-main threads

glfwSwapBuffers may be called by any thread but NSWindow may not.

Bug introduced by c3ca88055f and reported
by @crujose.

(cherry picked from commit 949275bbed)

src/cocoa_platform.h

@@ -121,6 +121,7 @@ typedef struct _GLFWwindowNS
     id              layer;
 
     GLFWbool        maximized;
+    GLFWbool        occluded;
     GLFWbool        retina;
 
     // Cached window properties to filter out duplicate events