Benchmark utilities

Together with @Xoppa I figured out a way to collect some benchmarking information like rendered tris of a `ModelBatch` or the total amount of gl calls that happened.

I think it might be useful to many people who want to do some performance optimizations, like reducing texture bindings by using texture atlasses or something similar.

There is a `Benchmark3DTest` which shows how the benchmark utilities are setup and how to get the information.

For now I measure the total gl calls, draw calls, texture bindings and shader switches (for this to work https://github.com/libgdx/libgdx/pull/1978 needs to be merged first). But this could be easily extended to also measure other special gl calls.