page: add caching from Plan 9

http://codereview.appspot.com/105070

src/cmd/page/page.c

@@ -228,6 +228,8 @@ threadmain(int argc, char **argv)
 		fprint(2, "page: initdraw failed: %r\n");
 		wexits("initdraw");
 	}
+	display->locking = 1;
+	
 	truecolor = screen->depth > 8;
 	viewer(doc);
 	wexits(0);