Describe the bug
Plotting undirected multigraphs with the Matplotlib backend, the resulting image has filled edges.

To reproduce
Use the following code:

import igraph as ig
import matplotlib.pyplot as plt

g = ig.Graph([(0,1), (1,0), (1,2), (2,0)], directed=False)

fig, ax = plt.subplots()
ig.plot(g, target=ax)

This generates the following image:

Version information
I am using version 0.11.2 installed using pip.