add borderless mode

Source/Engine/Platform/Linux/LinuxWindow.cpp

@@ -42,6 +42,15 @@ extern Array<KeyboardKeys> KeyCodeMap;
 extern X11::Cursor Cursors[(int32)CursorType::MAX];
 extern Window* MouseTrackingWindow;
 
+typedef struct XDisplayHints
+{
+    unsigned long flags;
+    unsigned long functions;
+    unsigned long decorations;
+    long inputMode;
+    unsigned long status;
+} XDisplayHints;
+
 static constexpr uint32 MouseDoubleClickTime = 500;
 static constexpr uint32 MaxDoubleClickDistanceSquared = 10;
 static X11::Time MouseLastButtonPressTime = 0;
@@ -332,6 +341,33 @@ void LinuxWindow::Maximize()
 	Maximize(true);
 }
 
+void LinuxWindow::SetBorderless(bool isBorderless, bool maximized)
+{
+    LINUX_WINDOW_PROLOG;
+    if (!display)
+        return;
+    XDisplayHints hints;
+    hints.flags = 2;
+    hints.decorations = isBorderless ? 0 : 1;
+    const X11::Atom property = X11::XInternAtom(display, "_MOTIF_WM_HINTS", true);
+    if (property != 0) {
+        X11::XChangeProperty(display, window, property, property, 32, PropModeReplace, (unsigned char *)&hints, 5);
+    }
+}
+
+void LinuxWindow::SetIsFullscreen(bool isFullscreen)
+{
+    if (isFullscreen)
+    {
+        SetBorderless(true);
+        Maximize(false);
+    } else
+    {
+        SetBorderless(false);
+        Restore();
+    }
+}
+
 void LinuxWindow::Restore()
 {
 	if (IsMaximized())