Add MGL entry points and port spinning_cube to use them

mojo/public/platform/native/mgl_onscreen_thunks.h

Index: mojo/public/platform/native/mgl_onscreen_thunks.h
diff --git a/mojo/public/platform/native/mgl_onscreen_thunks.h b/mojo/public/platform/native/mgl_onscreen_thunks.h
new file mode 100644
index 0000000000000000000000000000000000000000..40dcb807d373d88119bb863b387d4226bc54b62a
--- /dev/null
+++ b/mojo/public/platform/native/mgl_onscreen_thunks.h
@@ -0,0 +1,46 @@
+// 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.
+
+#ifndef MOJO_PUBLIC_PLATFORM_NATIVE_MGL_ONSCREEN_THUNKS_H_
+#define MOJO_PUBLIC_PLATFORM_NATIVE_MGL_ONSCREEN_THUNKS_H_
+
+#include <stddef.h>
+
+#include "mojo/public/c/gpu/MGL/mgl_onscreen.h"
+
+// Structure used to bind the interface which manipulates MGL contexts to a
+// DSO to theose of the embedder.
+//
+// This is the ABI between the embedder and the DSO. It can only have new
+// functions added to the end. No other changes are supported.
+#pragma pack(push, 8)
+struct MGLOnscreenThunks {
+  size_t size;  // Should be set to sizeof(MojoMGLOnscreenThunks).
+
+  void (*MGLResizeSurface)(uint32_t width, uint32_t height);
+  void (*MGLSwapBuffers)(void);
+};
+#pragma pack(pop)
+
+// Intended to be called from the embedder. Returns an object initialized to
+// contain pointers to each of the embedder's MGLOnscreenThunks functions.
+inline struct MGLOnscreenThunks MojoMakeMGLOnscreenThunks() {
+  struct MGLOnscreenThunks mgl_onscreen_thunks = {
+      sizeof(struct MGLOnscreenThunks), MGLResizeSurface, MGLSwapBuffers,
+  };
+
+  return mgl_onscreen_thunks;
+}
+
+// Use this type for the function found by dynamically discovering it in
+// a DSO linked with mojo_system. For example:
+// MojoSetMGLOnscreenThunksFn mojo_set_gles2_thunks_fn =
+//     reinterpret_cast<MojoSetMGLOnscreenThunksFn>(
+//         app_library.GetFunctionPointer("MojoSetMGLOnscreenThunks"));
+// The expected size of |mgl_thunks| is returned.
+// The contents of |mgl_thunks| are copied.
+typedef size_t (*MojoSetMGLOnscreenThunksFn)(
+    const struct MGLOnscreenThunks* mgl_thunks);
+
+#endif  // MOJO_PUBLIC_PLATFORM_NATIVE_MGL_ONSCREEN_THUNKS_H_