Plotting: multiple self-loops on a vertex are plotted on top of each other

[szhorvat]

When a vertex has multiple self-loops, they are plotted on top of each other. The graphs 1-1 and 1-1, 1-1 are not visually distinguishable when plotted.

g1 and g2 look the same:

library(igraph)
g1 <- make_graph(c(1,1))
g2 <- make_graph(c(1,1, 1,1))
plot(g1)
plot(g2)

Related: igraph/python-igraph#321 (see there for a suggestion about how this might be plotted nicely)