Add MGL entry points and port spinning_cube to use them

mojo/public/c/gpu/GLES2/gl2extmojo.h

 // Copyright (c) 2012 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.
 
-// This file contains Chromium-specific GLES2 extensions declarations.
+// This file contains Mojo-specific GLES2 extensions declarations.
 
-#ifndef GPU_GLES2_GL2EXTCHROMIUM_H_
-#define GPU_GLES2_GL2EXTCHROMIUM_H_
+#ifndef GPU_GLES2_GL2EXTMOJO_H_
+#define GPU_GLES2_GL2EXTMOJO_H_
 
 #include <GLES2/gl2.h>
 #include <GLES2/gl2ext.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
 /* GL_CHROMIUM_iosurface */
 #ifndef GL_CHROMIUM_iosurface
@@ skipping 759 matching lines @@
                                                         const GLfloat* m);
 typedef void(GL_APIENTRYP PFNGLMATRIXLOADIDENTITYCHROMIUMPROC)(
     GLenum matrixMode);
 
 #endif /* GL_CHROMIUM_path_rendering */
 
 #ifdef __cplusplus
 }
 #endif
 
-#endif  // GPU_GLES2_GL2EXTCHROMIUM_H_
+#endif  // GPU_GLES2_GL2EXTMOJO_H_