Add glfwWaitEventsTimeout

This function will put the calling thread to sleep until an event
arrives or until the specified timeout has elapsed.

src/cocoa_window.m

@@ -1214,6 +1214,19 @@ void _glfwPlatformWaitEvents(void)
     _glfwPlatformPollEvents();
 }
 
+void _glfwPlatformWaitEventsTimeout(double timeout)
+{
+    NSDate* date = [NSDate dateWithTimeIntervalSinceNow:timeout];
+    NSEvent* event = [NSApp nextEventMatchingMask:NSAnyEventMask
+                                        untilDate:date
+                                           inMode:NSDefaultRunLoopMode
+                                          dequeue:YES];
+    if (event)
+        [NSApp sendEvent:event];
+
+    _glfwPlatformPollEvents();
+}
+
 void _glfwPlatformPostEmptyEvent(void)
 {
     NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];