V3 triangles command

This pull request add Triangles command to support rendering of triangles.
quad command now is a subclass of trianglesCommand
Now VBO_SIZE is the number of vertices, which is the number of quad before. And the size of Vertex array is 65536, which is very big, we need to allocate a big array, but in most of situations, this is not needed.
Now the VBO_SIZE is 8192, which means that we can batch up to 2048 quads, which means 2048 sprites on the screen, I think it is enough.

We can change this back to 65536 if it is needed, what is your suggestions about this?