WIP: Visuals tests

Currently basic tests for different types of polygons, ellipses and arcs have been added.
Code de-duplication. You make the same calls 'get_test_file`, `imread`, and `assert_array_equal` in many places. That's one reason why @lcampagn suggested making a function in `vispy.testing` called `assert_image_equal()` that did these things. It would be something like `assert_image_equal(ss, 'visuals/circle1.png')`. Make sense?

1. Take a screenshot.

2. Test for image equality.