WIP: Visuals tests

Currently basic tests for different types of polygons, ellipses and arcs have been added.
I think `assert_image_equal` is actually more general. For example, I could easily use your functions in `vispy.fonts` as:

```
from vispy.fonts import _load_glyph
from vispy.testing import assert_image_equal

def test_glyph_bitmap():
    """Test glyph creation"""
    arial_a = _load_glyph(...)
    assert_image_equal(arial_a['bitmap'], 'fonts/arial_a.png')
```
See what I mean? The function is about comparing reference images to anything else of interest (most of which might happen to be visuals, but not necessarily all)