Automated rendering tests

[c42f]

As discussed at JuliaLang/METADATA.jl#6436 (review) it would be nice to have some rendering tests running in travis, though might be tricky. Some relevant parts of the discussion:

Testing in a virtualized headless environment will be "interesting" as it normally depends on a working X windows and working OpenGL 3.3 implementation to actually do anything useful. Steps for an end-to-end test could be something like:

• Ensure displaz-gui can even start in a headless environment (looks possible via xvfb https://docs.travis-ci.com/user/gui-and-headless-browsers/ ?)
• Ensure we can get an OpenGL 3.3 context on travis (dubious? OpenGL 3+ support travis-ci/travis-ci#5645 )
• Implement a proper scriptable screenshot command (semi-working one at Screenshot Command #98 )
• Create test data for displaz-gui to send via a shell script, point the camera somewhere, etc.
• Compare screenshots against reference images with an image diff algorithm of some kind.