Add MGL entry points and port spinning_cube to use them

mojo/public/c/gpu/MGL/mgl_onscreen.h

+// 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.
+
+// Note: This header should be compilable as C.
+
+#ifndef MOJO_PUBLIC_C_GPU_MGL_MGL_ONSCREEN_H_
+#define MOJO_PUBLIC_C_GPU_MGL_MGL_ONSCREEN_H_
+
+#include <stdint.h>
+
+#include "mojo/public/c/gpu/MGL/mgl_types.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Resizes the default framebuffer for the currently bound onscreen MGLContext.
+void MGLResizeSurface(uint32_t width, uint32_t height);
+
+// Presents the default framebuffer for the currently bound onscreen MGLContext
+// to the windowing system or display.
+void MGLSwapBuffers();
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+#endif  // MOJO_PUBLIC_C_GPU_MGL_MGL_ONSCREEN_H_