Fix axvspan for drawing slices on polar plots.

[anntzer]

There's already special-casing for polar plots in bar() (using the same
_interpolation_steps = 100); it seems reasonable to use the same
approach for axvspan (e.g. polar(); axvspan(0, pi/4)).

It seems like making axhspan work (basically drawing full rings) is less
easy, but there's an easy enough workaround
(polar(); bar(0, bottom=1, height=1, width=2*pi)) as the "x" (theta)
extent is actually known in that case.

plt.polar(); plt.axvspan(np.pi/4, np.pi/2)
before:

after:

PR Summary

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