Fixed viewport transparent bg when clear mode is enabled.

servers/visual/visual_server_viewport.cpp

@@ -54,7 +54,7 @@ void VisualServerViewport::_draw_viewport(Viewport *p_viewport, ARVRInterface::E
 	bool can_draw_3d = !p_viewport->disable_3d && !p_viewport->disable_3d_by_usage && VSG::scene->camera_owner.owns(p_viewport->camera);
 
 	if (p_viewport->clear_mode != VS::VIEWPORT_CLEAR_NEVER) {
-		VSG::rasterizer->clear_render_target(clear_color);
+		VSG::rasterizer->clear_render_target(p_viewport->transparent_bg ? Color(0, 0, 0, 0) : clear_color);
 		if (p_viewport->clear_mode == VS::VIEWPORT_CLEAR_ONLY_NEXT_FRAME) {
 			p_viewport->clear_mode = VS::VIEWPORT_CLEAR_NEVER;
 		}
@@ -504,6 +504,7 @@ void VisualServerViewport::viewport_set_transparent_background(RID p_viewport, b
 	ERR_FAIL_COND(!viewport);
 
 	VSG::storage->render_target_set_flag(viewport->render_target, RasterizerStorage::RENDER_TARGET_TRANSPARENT, p_enabled);
+	viewport->transparent_bg = true;
 }
 
 void VisualServerViewport::viewport_set_global_canvas_transform(RID p_viewport, const Transform2D &p_transform) {

servers/visual/visual_server_viewport.h

@@ -72,6 +72,8 @@ public:
 
 		VS::ViewportClearMode clear_mode;
 
+		bool transparent_bg;
+
 		struct CanvasKey {
 
 			int layer;
@@ -101,6 +103,7 @@ public:
 		Viewport() {
 			update_mode = VS::VIEWPORT_UPDATE_WHEN_VISIBLE;
 			clear_mode = VS::VIEWPORT_CLEAR_ALWAYS;
+			transparent_bg = false;
 			disable_environment = false;
 			viewport_to_screen = 0;
 			shadow_atlas_size = 0;