Wayland: Fix glfwPostEmptyEvent not always working

The display sync requests in glfwPostEmptyEvent could just accumulate as
the display was never flushed on secondary threads.

This adds a proper flush after each sync request.

Fixes #1520
Closes #1521

(cherry picked from commit a32cbf6d4f)

src/wl_window.c

@@ -1393,6 +1393,7 @@ void _glfwPlatformWaitEventsTimeout(double timeout)
 void _glfwPlatformPostEmptyEvent(void)
 {
     wl_display_sync(_glfw.wl.display);
+    flushDisplay();
 }
 
 void _glfwPlatformGetCursorPos(_GLFWwindow* window, double* xpos, double* ypos)