Preserve path breaks during interpolation

[has2k1]

Under a non-linear coordinate system, a geom_polygon with several groups draws an edge from one polygon to the next, and separate geom_path groups join up when an aesthetic varies along the line.

df = pd.DataFrame({
    "x": [1, 2, 2, 1, 3, 4, 4, 3],
    "y": [1, 1, 2, 2, 3, 3, 4, 4],
    "g": list("aaaabbbb"),
})

(
    ggplot(df, aes("x", "y", fill="g"))
    + geom_polygon(colour="black", size=1)
    + coord_trans(x="log10", y="log10")
)

Without this PR, a diagonal edge runs from the bottom-left corner of the first square to the second square.