Immediate Mode / VertBatch performance could be improved by reusing gl buffers

[richardeakin]

After reading this discussion that compares the performance of drawing in immediate mode in 0.8.6 and glNext, I did some profiling to find out where the slow down was. For me using Instruments / Time Profiler to be > 50% in the destruction of the Vbo, and also another 5% in destroying the VAO:

So, I did a quick hack (see here) so that those gl buffers are reused, and I saw a significant performance improvement:

While still not as good as 0.8.6, and would surely take a bit more thought to cover all of the cases, it seems worth the effort in that using something like gl::begin( GL_LINES ) is very common - we even use this in BasicApp, pretty much the first piece of cinder sample code that a new user will see.

Test app is the same as in the discussion, from this gist.