Cocoa: user performSelectorOnMainThread instead of postEvent for the tick callback

performSelectorOnMainThread runs in more loop run modes which means that
the tick callback will behave more like it does on other platforms,
during window resizes and other modal event loops.

kitty/glfw.c

@@ -939,8 +939,8 @@ void
 wakeup_main_loop() {
     request_tick_callback();
 #ifndef __APPLE__
-    // On Cocoa request_tick_callback() uses an event which wakes up the
-    // main loop anyway
+    // On Cocoa request_tick_callback() uses performSelectorOnMainLoop which
+    // wakes up the main loop anyway
     glfwPostEmptyEvent();
 #endif
 }