Add MGL entry points and port spinning_cube to use them

examples/spinning_cube/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//mojo/public/mojo_application.gni")
 
 mojo_native_application("spinning_cube") {
   sources = [
     "gles2_client_impl.cc",
     "gles2_client_impl.h",
     "spinning_cube_app.cc",
   ]
 
   deps = [
     ":lib",
     "//base",
     "//mojo/public/c/gles2",
+    "//mojo/public/c/gpu",
+    "//mojo/public/c/gpu:gpu_onscreen",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/environment",
     "//mojo/public/cpp/system",
     "//mojo/public/cpp/utility",
     "//mojo/services/geometry/public/interfaces",
     "//mojo/services/native_viewport/public/interfaces",
   ]
 }
 
 source_set("lib") {
   sources = [
     "spinning_cube.cc",
     "spinning_cube.h",
   ]
 
   deps = [
     "//base",
     "//mojo/common",
-    "//mojo/public/c/gles2",
+    "//mojo/public/c/gpu",
     "//mojo/public/cpp/environment",
   ]
 }