Implement show/hide_cursor() under Linux

src/os/linux/Input.cpp

@@ -157,11 +157,13 @@ void set_cursor_xy(int32_t x, int32_t y)
 //-----------------------------------------------------------------------------
 void hide_cursor()
 {
+	XDefineCursor(display, window, x11_hidden_cursor);
 }
 
 //-----------------------------------------------------------------------------
 void show_cursor()
 {
+	XDefineCursor(display, window, None);
 }
 
 //-----------------------------------------------------------------------------