Add thread guard for force_draw and update relared doc

force_draw must be called from main thread

doc/classes/RenderingServer.xml

@@ -1362,7 +1362,7 @@
 			<param index="0" name="swap_buffers" type="bool" default="true" />
 			<param index="1" name="frame_step" type="float" default="0.0" />
 			<description>
-				Forces redrawing of all viewports at once.
+				Forces redrawing of all viewports at once. Must be called from the main thread.
 			</description>
 		</method>
 		<method name="force_sync">

servers/rendering/rendering_server_default.cpp

@@ -379,6 +379,7 @@ void RenderingServerDefault::sync() {
 }
 
 void RenderingServerDefault::draw(bool p_swap_buffers, double frame_step) {
+	ERR_FAIL_COND_MSG(!Thread::is_main_thread(), "Manually triggering the draw function from the RenderingServer can only be done on the main thread. Call this function from the main thread or use call_deferred().");
 	if (create_thread) {
 		command_queue.push(this, &RenderingServerDefault::_thread_draw, p_swap_buffers, frame_step);
 	} else {