VNC: Add control logic to negotiate security and framebuffer

graphics/vnc/server/vnc_server.c

@@ -120,17 +120,27 @@ int vnc_server(int argc, FAR char *argv[])
         {
           gvdbg("New VNC connection\n");
 
-          /* Start the VNC session.  This function does not return until the
+          ret = vnc_negotiate(session);
-           * session has been terminated (or an error occurs).
+          if (ret < 0)
+            {
+              gdbg("ERROR: Failed to negotiate security/framebuffer: %d\n",
+                   ret);
+            }
+          else
+            {
+              /* Start the VNC session.  This function does not return until
+               * the session has been terminated (or an error occurs).
                */
 
-          (void)vnc_session(session);
+              ret = vnc_session(session);
+              gvdbg("Session terminated with %d\n", ret);
+            }
+        }
 
       /* Re-initialize the session structure for re-use */
 
       vnc_release_session(session);
     }
-    }
 
   return EXIT_FAILURE; /* We won't get here */
 }

graphics/vnc/server/vnc_server.h

@@ -250,6 +250,25 @@ FAR struct vnc_session_s *vnc_create_session(void);
 
 void vnc_release_session(FAR struct vnc_session_s *session);
 
+/****************************************************************************
+ * Name: vnc_negotiate
+ *
+ * Description:
+ *  Perform the VNC initialize sequency after a client has sucessfully
+ *  connected to the server.  Negotiate security, framebuffer and color
+ *  properties.
+ *
+ * Input Parameters:
+ *   session - An instance of the session structure allocated by
+ *     vnc_create_session().
+ *
+ * Returned Value:
+ *   Returns zero (OK) on success; a negated errno value on failure.
+ *
+ ****************************************************************************/
+
+int vnc_negotiate(FAR struct vnc_session_s *session);
+
 /****************************************************************************
  * Name: vnc_session
  *