Vbo utils

Add a VertexLayout class and VboMesh class, then add an example use case to the Tangram test program
The draw call won't function properly unless the buffer is bound each time (it must be bound when the vertex layout ```enable``` call enables the vertex attributes). There is the possibility of some minimal duplication if ```upload()``` is called in this method above (if the mesh wasn't already explicitly uploaded before the draw call), but that would only occur once and would be negligible.