Fix certain resource previews empty with GL compat

As an additional fix, the preview render request now ignores the main viewport hierarchy so previews don't pay the cost of re-rendering the whole editor and descendant viewports.

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 "scene/main/window.h"
 #include "scene/resources/image_texture.h"
 #include "servers/rendering/rendering_server_default.h"
 
@@ -96,14 +97,25 @@ EditorResourcePreviewGenerator::EditorResourcePreviewGenerator() {
 }
 
 void EditorResourcePreviewGenerator::DrawRequester::request_and_wait(RID p_viewport) const {
+	Callable request_vp_update_once = callable_mp(RS::get_singleton(), &RS::viewport_set_update_mode).bind(p_viewport, RS::VIEWPORT_UPDATE_ONCE);
+
 	if (EditorResourcePreview::get_singleton()->is_threaded()) {
-		Callable request_vp_update_once = callable_mp(RS::get_singleton(), &RS::viewport_set_update_mode).bind(p_viewport, RS::VIEWPORT_UPDATE_ONCE);
 		RS::get_singleton()->connect(SNAME("frame_pre_draw"), request_vp_update_once, Object::CONNECT_ONE_SHOT);
 		RS::get_singleton()->request_frame_drawn_callback(callable_mp(const_cast<EditorResourcePreviewGenerator::DrawRequester *>(this), &EditorResourcePreviewGenerator::DrawRequester::_post_semaphore));
 
 		semaphore.wait();
 	} else {
+		// Avoid the main viewport and children being redrawn.
+		SceneTree *st = Object::cast_to<SceneTree>(OS::get_singleton()->get_main_loop());
+		ERR_FAIL_NULL_MSG(st, "Editor's MainLoop is not a SceneTree. This is a bug.");
+		RID root_vp = st->get_root()->get_viewport_rid();
+		RenderingServer::get_singleton()->viewport_set_active(root_vp, false);
+
+		request_vp_update_once.call();
 		RS::get_singleton()->draw(false);
+
+		// Let main viewport and children be drawn again.
+		RenderingServer::get_singleton()->viewport_set_active(root_vp, true);
 	}
 }