Wayland: Fix toggling of server-side decorations

This is a temporary local fix to have updates to GLFW_DECORATED mostly
work as intended.  The whole decoration state machine needs to be
restructured, but not by this commit.

README.md

@@ -346,6 +346,8 @@ information on what to include when reporting a bug.
  - [Wayland] Bugfix: Manual resizing with fallback decorations behaved erratically
    (#1991,#2115,#2127)
  - [Wayland] Bugfix: Size limits included frame size for fallback decorations
+ - [Wayland] Bugfix: Updating `GLFW_DECORATED` had no effect on server-side
+   decorations
  - [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

@@ -2097,12 +2097,26 @@ void _glfwSetWindowResizableWayland(_GLFWwindow* window, GLFWbool enabled)
 }
 
 void _glfwSetWindowDecoratedWayland(_GLFWwindow* window, GLFWbool enabled)
+{
+    if (window->wl.xdg.decoration)
+    {
+        uint32_t mode;
+
+        if (enabled)
+            mode = ZXDG_TOPLEVEL_DECORATION_V1_MODE_SERVER_SIDE;
+        else
+            mode = ZXDG_TOPLEVEL_DECORATION_V1_MODE_CLIENT_SIDE;
+
+        zxdg_toplevel_decoration_v1_set_mode(window->wl.xdg.decoration, mode);
+    }
+    else
     {
         if (enabled)
             createDecorations(window);
         else
             destroyDecorations(window);
     }
+}
 
 void _glfwSetWindowFloatingWayland(_GLFWwindow* window, GLFWbool enabled)
 {