WIP: Visuals tests

Currently basic tests for different types of polygons, ellipses and arcs have been added.
Instead of doing this, you should override the `__enter__` method of `Canvas`. That is the one that calls `_vispy_warmup` and ensures that the Canvas is "good to go". So you want something like:
```
def __enter__(self):
    Canvas.__enter__(self)
    from .. import gloo
    gloo.clear(color=self._clear_color)
    gloo.set_viewport(0, 0, *self.size)
    return self
```
Make sense?