Vbo utils

Add a VertexLayout class and VboMesh class, then add an example use case to the Tangram test program
Yup

reset has the following function:
* if the pointer contained in the unique_ptr is nullptr, it reassigns the new pointer to this unique_ptr.
* if the pointer contained in the unique_ptr is not nullptr, it calls the delete() for the old_ptr and either assigns this unique_ptr to a new pointer if one is specified (using new operator) or makes it nullptr if no new is specified.

So in this case, since previously layout has nothing defined (line 40), i.e. contains a nullptr, first case is used and layout assigns "new VertexLayout" as its contents.