This fixes a number of things that are warnings in the Mac compiler....

core/cross/gl/buffer_gl.cc

 /*
  * Copyright 2009, Google Inc.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 39 matching lines @@
 namespace {
 
 GLenum BufferAccessModeToGLenum(Buffer::AccessMode access_mode) {
   switch (access_mode) {
     case Buffer::READ_ONLY:
       return GL_READ_ONLY_ARB;
     case Buffer::WRITE_ONLY:
       return GL_WRITE_ONLY_ARB;
     case Buffer::READ_WRITE:
       return GL_READ_WRITE_ARB;
+    case Buffer::NONE:
+      break;
   }
   DCHECK(false);
   return GL_READ_WRITE_ARB;
 }
 
 }  // anonymous namespace
 
 // Vertex Buffers --------------------------------------------------------------
 
 // Initializes the O3D VertexBuffer object but does not allocate an
@@ skipping 173 matching lines @@
     } else {
       O3D_ERROR(
           service_locator()) << "Unable to unlock a GL Element Array Buffer";
     }
     return false;
   }
   CHECK_GL_ERROR();
   return true;
 }
 }  // namespace o3d