const qualifiers on MeshBatch::add arguments

Since MeshBatch::add doesn't modify incoming vertices and indices, it's better to use const qualifier on these arguments. In this case MeshBatch::add could also be called from const-methods. Suppose, for example, the following method definition:
```C++
void Foo::Render( ) const
{
    _meshBatch->add( &( *_vertices.begin( ) ), _vertices.size( ) );
}
```
where _vertices is of type std::vector and is a member of Foo. Without const qualifier on MeshBatch::add's arguments this code fragment will require const_cast on _vertices.begin( )