EditorResourcePreview: Let loads complete after exit requested

(cherry picked from commit 213fda4)

editor/editor_node.cpp

@@ -3353,12 +3353,13 @@ int EditorNode::_next_unsaved_scene(bool p_valid_filename, int p_start) {
 
 void EditorNode::_exit_editor(int p_exit_code) {
 	exiting = true;
-	resource_preview->stop(); // Stop early to avoid crashes.
 	_save_editor_layout();
 
 	// Dim the editor window while it's quitting to make it clearer that it's busy.
 	dim_editor(true);
 
+	resource_preview->stop();
+
 	// Unload addons before quitting to allow cleanup.
 	unload_editor_addons();
 

editor/editor_resource_preview.cpp

@@ -40,6 +40,7 @@
 #include "editor/editor_settings.h"
 #include "editor/editor_string_names.h"
 #include "editor/themes/editor_scale.h"
+#include "main/main.h"
 #include "scene/main/window.h"
 #include "scene/resources/image_texture.h"
 #include "servers/rendering/rendering_server_default.h"
@@ -531,8 +532,8 @@ void EditorResourcePreview::stop() {
 			}
 
 			while (!exited.is_set()) {
-				OS::get_singleton()->delay_usec(10000);
-				RenderingServer::get_singleton()->sync(); //sync pending stuff, as thread may be blocked on rendering server
+				// Sync pending work (message queue, render).
+				Main::iteration();
 			}
 
 			thread.wait_to_finish();