Add MGL entry points and port spinning_cube to use them

mojo/public/platform/native/mgl_thunks.h

Index: mojo/public/platform/native/mgl_thunks.h
diff --git a/mojo/public/platform/native/mgl_thunks.h b/mojo/public/platform/native/mgl_thunks.h
new file mode 100644
index 0000000000000000000000000000000000000000..8a56474ea03227850cca9797bfe149b266268f7b
--- /dev/null
+++ b/mojo/public/platform/native/mgl_thunks.h
@@ -0,0 +1,56 @@
+// 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_THUNKS_H_
+#define MOJO_PUBLIC_PLATFORM_NATIVE_MGL_THUNKS_H_
+
+#include <stddef.h>
+
+#include "mojo/public/c/gpu/MGL/mgl.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 MGLThunks {
+  size_t size;  // Should be set to sizeof(MGLThunks).
+
+  MGLContext (*MGLCreateContext)(MGLOpenGLAPIVersion version,
+                                 MojoHandle command_buffer_handle,
+                                 MGLContext share_group,
+                                 MGLContextLostCallback lost_callback,
+                                 void* lost_callback_closure,
+                                 const struct MojoAsyncWaiter* async_waiter);
+  void (*MGLDestroyContext)(MGLContext context);
+  void (*MGLMakeCurrent)(MGLContext context);
+  MGLContext (*MGLGetCurrentContext)(void);
+};
+#pragma pack(pop)
+
+// Intended to be called from the embedder. Returns an object initialized to
+// contain pointers to each of the embedder's MGLThunks functions.
+inline struct MGLThunks MojoMakeMGLThunks() {
+  struct MGLThunks mgl_thunks = {
+      sizeof(struct MGLThunks),
+      MGLCreateContext,
+      MGLDestroyContext,
+      MGLMakeCurrent,
+      MGLGetCurrentContext,
+  };
+
+  return mgl_thunks;
+}
+
+// Use this type for the function found by dynamically discovering it in
+// a DSO linked with mojo_system. For example:
+// MojoSetMGLThunksFn mojo_set_gles2_thunks_fn =
+//     reinterpret_cast<MojoSetMGLThunksFn>(
+//         app_library.GetFunctionPointer("MojoSetMGLThunks"));
+// The expected size of |mgl_thunks| is returned.
+// The contents of |mgl_thunks| are copied.
+typedef size_t (*MojoSetMGLThunksFn)(const struct MGLThunks* mgl_thunks);
+
+#endif  // MOJO_PUBLIC_PLATFORM_NATIVE_MGL_THUNKS_H_