Fix crash when switching to new workspace during transaction

When rendering, the workspace for the output needs to be retrieved from
the output's `current` state. output_get_active_workspace returns the
pending workspace, which crashes if the pending workspace is new and
hasn't completed a transaction yet.

sway/desktop/render.c

@@ -871,7 +871,7 @@ void output_render(struct sway_output *output, struct timespec *when,
 		pixman_region32_union_rect(damage, damage, 0, 0, width, height);
 	}
 
-	struct sway_workspace *workspace = output_get_active_workspace(output);
+	struct sway_workspace *workspace = output->current.active_workspace;
 	struct sway_container *fullscreen_con = workspace->current.fullscreen;
 
 	if (output_has_opaque_overlay_layer_surface(output)) {