Add the source of a very simple program using OpenGL through GLX.

We'll use this program to generate traces for use in the test suite,
(to exercise using "apitrace trim" to trim away unused textures,
shader programs, etc.).

There is a Makefile here, for convenience to any test writers wanting
to expand this program. Note that this Makefile us intentionally not
integrated with the cmake setup of the apitrace-tests project. That's
because it is not expected that this test program should be built or
run as a standard part of the test suite. Instead, useful trace files
generated from glxsimple will be directly committed to the
apitrace-tests repository.