Wayland: Fix mapping failure on wlroots compositor

When showing a window that had already been shown once (and so already
had its shell objects), GLFW would attach a new buffer and commit it
before waiting for the next configure event.  This was a violation of
the XDG shell protocol.

This was allowed to work as intended on GNOME and KDE without error.
However wlroots based compositors would (correctly) emit an error.

Unfortunately, I haven't been able to find a way to get both KDE, GNOME
and Sway to send the configure event we need in order to map the
wl_surface again while keeping our existing shell objects, so with this
commit we now create them for each call to glfwShowWindow and destroy
them for each call to glfwHideWindow.

Fixes #1268

(cherry picked from commit 83a134a92f)

README.md

@@ -178,6 +178,8 @@ information on what to include when reporting a bug.
    applying the specified ratio
  - [Wayland] Bugfix: `GLFW_MAXIMIZED` window hint had no effect
  - [Wayland] Bugfix: `glfwRestoreWindow` had no effect before first show
+ - [Wayland] Bugfix: Hiding and then showing a window caused program abort on
+   wlroots compositors (#1268)
 
 
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