Added caching of X11 NULL atom.

src/x11_clipboard.c

@@ -139,7 +139,7 @@ static Atom writeTargetToProperty(const XSelectionRequestEvent* request)
         XChangeProperty(_glfw.x11.display,
                         request->requestor,
                         request->property,
-                        XInternAtom(_glfw.x11.display, "NULL", False),
+                        _glfw.x11._NULL,
                         32,
                         PropModeReplace,
                         NULL,

src/x11_init.c

@@ -524,6 +524,7 @@ static GLboolean initExtensions(void)
     detectEWMH();
 
     // Find or create string format atoms
+    _glfw.x11._NULL = XInternAtom(_glfw.x11.display, "NULL", False);
     _glfw.x11.UTF8_STRING =
         XInternAtom(_glfw.x11.display, "UTF8_STRING", False);
     _glfw.x11.COMPOUND_STRING =

src/x11_platform.h

@@ -136,6 +136,7 @@ typedef struct _GLFWlibraryX11
     Atom            CLIPBOARD;
     Atom            CLIPBOARD_MANAGER;
     Atom            SAVE_TARGETS;
+    Atom            _NULL;
     Atom            UTF8_STRING;
     Atom            COMPOUND_STRING;
     Atom            ATOM_PAIR;