[tests] Adds an interactive cogl vertex buffer unit test

[tests] Adds an interactive cogl vertex buffer unit test

The test is a sanity check that dynamic updating of vertex data via the cogl
vertex buffer api works and has reasonable performance. (though it can't be
considered a well designed benchmark since it wastes casual amounts of CPU
time simply choosing pretty colors.)

The code also aims to demonstrate one way of creating, updating and efficiently
drawing a quad mesh structure via the vertex buffer api which could be applied
to lots of different use cases.