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

README.md

@@ -298,6 +298,7 @@ information on what to include when reporting a bug.
  - [Wayland] Bugfix: Full screen window creation did not ignore `GLFW_VISIBLE`
  - [Wayland] Bugfix: Some keys were reported as wrong key or `GLFW_KEY_UNKNOWN`
  - [Wayland] Bugfix: Text input did not repeat along with key repeat
+ - [Wayland] Bugfix: `glfwPostEmptyEvent` sometimes had no effect (#1520,#1521)
  - [POSIX] Removed use of deprecated function `gettimeofday`
  - [POSIX] Bugfix: `CLOCK_MONOTONIC` was not correctly tested for or enabled
  - [WGL] Disabled the DWM swap interval hack for Windows 8 and later (#1072)

src/wl_window.c

@@ -1185,6 +1185,7 @@ void _glfwWaitEventsTimeoutWayland(double timeout)
 void _glfwPostEmptyEventWayland(void)
 {
     wl_display_sync(_glfw.wl.display);
+    flushDisplay();
 }
 
 void _glfwGetCursorPosWayland(_GLFWwindow* window, double* xpos, double* ypos)