Vbo utils

Add a VertexLayout class and VboMesh class, then add an example use case to the Tangram test program
Is putting the * in the pointer with the type our official style? I originally learned to put it with the variable name, e.g. ```GLvoid *offset```, so it always looks slightly off to me -- but it does make more sense with the type, since it is more like "type" information, so I'm down with this, just checking since I didn't remember discussing.