GLFW: Add support for window occluded notifications on macOS

Based on: https://github.com/glfw/glfw/pull/1123

glfw/cocoa_window.m

@@ -425,6 +425,11 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
     _glfwInputWindowPos(window, x, y);
 }
 
+- (void)windowDidChangeOcclusionState:(NSNotification *)notification
+{
+    _glfwInputWindowOcclusion(window, !([window->ns.object occlusionState] & NSWindowOcclusionStateVisible));
+}
+
 - (void)windowDidMiniaturize:(NSNotification *)notification
 {
     if (window->monitor)
@@ -1671,6 +1676,11 @@ int _glfwPlatformWindowFocused(_GLFWwindow* window)
     return [window->ns.object isKeyWindow];
 }
 
+int _glfwPlatformWindowOccluded(_GLFWwindow* window)
+{
+    return !([window->ns.object occlusionState] & NSWindowOcclusionStateVisible);
+}
+
 int _glfwPlatformWindowIconified(_GLFWwindow* window)
 {
     return [window->ns.object isMiniaturized];