X11: Fix decoration enabling after window creation

This fixes the enabling of window decorations after creation.  Instead
of removing the _MOTIF_WM_HINTS property, we now set or unset the
MWM_DECOR_ALL bit of the decorations field.

Fixes #1566.

(cherry picked from commit 5fc4c01302)

README.md

@@ -132,6 +132,7 @@ information on what to include when reporting a bug.
  - [X11] Bugfix: The CMake files did not check for the XInput headers (#1480)
  - [X11] Bugfix: Key names were not updated when the keyboard layout changed
    (#1462,#1528)
+ - [X11] Bugfix: Decorations could not be enabled after window creation (#1566)
 
 
 ## Contact

src/x11_window.c

@@ -50,6 +50,10 @@
 #define Button6            6
 #define Button7            7
 
+// Motif WM hints flags
+#define MWM_HINTS_DECORATIONS   2
+#define MWM_DECOR_ALL           1
+
 #define _GLFW_XDND_VERSION 5
 
 
@@ -2536,14 +2540,6 @@ void _glfwPlatformSetWindowResizable(_GLFWwindow* window, GLFWbool enabled)
 
 void _glfwPlatformSetWindowDecorated(_GLFWwindow* window, GLFWbool enabled)
 {
-    if (enabled)
-    {
-        XDeleteProperty(_glfw.x11.display,
-                        window->x11.handle,
-                        _glfw.x11.MOTIF_WM_HINTS);
-    }
-    else
-    {
     struct
     {
         unsigned long flags;
@@ -2551,10 +2547,10 @@ void _glfwPlatformSetWindowDecorated(_GLFWwindow* window, GLFWbool enabled)
         unsigned long decorations;
         long input_mode;
         unsigned long status;
-        } hints;
+    } hints = {0};
 
-        hints.flags = 2;       // Set decorations
+    hints.flags = MWM_HINTS_DECORATIONS;
-        hints.decorations = 0; // No decorations
+    hints.decorations = enabled ? MWM_DECOR_ALL : 0;
 
     XChangeProperty(_glfw.x11.display, window->x11.handle,
                     _glfw.x11.MOTIF_WM_HINTS,
@@ -2562,7 +2558,6 @@ void _glfwPlatformSetWindowDecorated(_GLFWwindow* window, GLFWbool enabled)
                     PropModeReplace,
                     (unsigned char*) &hints,
                     sizeof(hints) / sizeof(long));
-    }
 }
 
 void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)