Vbo utils

Add a VertexLayout class and VboMesh class, then add an example use case to the Tangram test program
Haha I'll add a comment in code! This explicitly reinterprets the bits in m_stride (which, at this line, is the offset of the current attribute in bytes) as a void pointer. This differs from a c-style cast in that so-called "narrowing" conversions are allowed without warnings.