FS-7500: add ability to insert a custom callback to the core video thread

the callback will be called on each loop on read video frame, or
the callback function call run it's own loop to take over the core
loop so it can read video from session by itself.

the callback function can -

return SWITCH_STATUS_SUCCESS to wait another loop
return SWITCH_STATUS_CONTINUE to continue use the default behaviour
return anything else will break the core video loop and end the
       core thread

src/include/switch_core.h

@@ -2303,6 +2303,35 @@ SWITCH_DECLARE(uint8_t) switch_core_session_compare(switch_core_session_t *a, sw
   \return TRUE or FALSE
 */
 SWITCH_DECLARE(uint8_t) switch_core_session_check_interface(switch_core_session_t *session, const switch_endpoint_interface_t *endpoint_interface);
+
+/*!
+  \brief Set a callback to let the core video thread call us
+  \param session the session
+  \param func to callback
+  \param private user data
+  \return SWITCH_STATUS_CONTINUE | SWITCH_STATUS_SUCCESS | SWITCH_STATUS_BREAK | SWITCH_STATUS_*
+
+  If returns SWITCH_STATUS_CONTINUE, it will continues to run furthur code (read/write) in the core video thread,
+  that is to say, if the callback func to nothing and just returns SWITCH_STATUS_CONTINUE, it remains the default behaviour,
+  Return SWITCH_STATUS_SUCCESS to skip the default behaviour
+  Return SWITCH_STATUS_BREAK will break the loop and end the video thread
+*/
+
+SWITCH_DECLARE(switch_status_t) switch_core_session_set_video_thread_callback(switch_core_session_t *session, void *func, void *user_data);
+
+/*!
+  \brief Set a callback to let the core video thread call us
+  \param session the session
+  \param the current video frame
+  \param private user data
+  \return SWITCH_STATUS_CONTINUE or SWITCH_STATUS_SUCCESS
+
+  If returns SWITCH_STATUS_CONTINUE, it will continues to run furthur code (read/write) in the core video thread,
+  that is to say, if the callback func to nothing and just returns SWITCH_STATUS_CONTINUE, it remains the default behaviour,
+  Return SWITCH_STATUS_SUCCESS to skip the default behaviour
+*/
+SWITCH_DECLARE(switch_status_t) switch_core_session_video_thread_callback(switch_core_session_t *session, switch_frame_t *frame);
+
 SWITCH_DECLARE(switch_hash_index_t *) switch_core_mime_index(void);
 SWITCH_DECLARE(const char *) switch_core_mime_ext2type(const char *ext);
 SWITCH_DECLARE(const char *) switch_core_mime_type2ext(const char *type);