Vbo utils

Add a VertexLayout class and VboMesh class, then add an example use case to the Tangram test program
If I understand what you're saying, that would generate more total GL calls to enable and disable. Those calls (or really any GL calls, which generate driver overhead) are likely to be far more expensive (at least that's my understanding... it's unknown how much the driver from device to device will efficiently cache and avoid redundant operations).