Add MGL entry points and port spinning_cube to use them

mojo/public/platform/native/mgl_onscreen_thunks.c

+// Copyright 2015 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.
+
+#include "mojo/public/platform/native/mgl_onscreen_thunks.h"
+
+#include <assert.h>
+
+#include "mojo/public/platform/native/thunk_export.h"
+
+static struct MGLOnscreenThunks g_onscreen_thunks = {0};
+
+void MGLResizeSurface(uint32_t width, uint32_t height) {
+  assert(g_onscreen_thunks.MGLResizeSurface);
+  g_onscreen_thunks.MGLResizeSurface(width, height);
+}
+
+void MGLSwapBuffers(void) {
+  assert(g_onscreen_thunks.MGLSwapBuffers);
+  g_onscreen_thunks.MGLSwapBuffers();
+}
+
+THUNK_EXPORT size_t MojoSetMGLOnscreenThunks(
+    const struct MGLOnscreenThunks* mgl_onscreen_thunks) {
+  if (mgl_onscreen_thunks->size >= sizeof(g_onscreen_thunks))
+    g_onscreen_thunks = *mgl_onscreen_thunks;
+  return sizeof(g_onscreen_thunks);
+}