Wayland: Fix missing damage event on window show

By definition a hidden window on Wayland does not have valid framebuffer
contents.

This adds a window damage (refresh) event when a window is shown, to
request an initial frame for the now visible window.

README.md

@@ -284,6 +284,7 @@ information on what to include when reporting a bug.
  - [Wayland] Bugfix: Disable key repeat mechanism when window loses input focus
  - [Wayland] Bugfix: Window hiding and showing did not work (#1492,#1731)
  - [Wayland] Bugfix: A key being repeated was not released when window lost focus
+ - [Wayland] Bugfix: Showing a hidden window did not emit a window refresh event
  - [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

@@ -1006,6 +1006,7 @@ void _glfwShowWindowWayland(_GLFWwindow* window)
             createXdgSurface(window);
 
         window->wl.visible = GLFW_TRUE;
+        _glfwInputWindowDamage(window);
     }
 }