Wrong zorder of land background

[zxdawn]

Description

If we plot the data first and then add the landcolor, the land will cover the data. It's better to set the land to the lowest zorder.

Steps to reproduce

import proplot as pplt
import numpy as np

offset = -40
lon = pplt.arange(offset, 360 + offset - 1, 60)
lat = pplt.arange(60, 70, 1)
state = np.random.RandomState(51423)
data = state.rand(len(lat), len(lon))

fig, axs = pplt.subplots(proj='npstere')
axs.pcolor(lon, lat, data, cmap='viridis')
axs.format(boundinglat=60, land=True, landcolor='gray5')

Expected behavior:
Format axis before plot data:

axs.format(boundinglat=60, land=True, landcolor='gray5')
axs.pcolor(lon, lat, data, cmap='viridis')

Actual behavior:

Proplot version

3.5.1
0.9.5.post202

[zxdawn]

Note that the coast should be kept as the highest zorder.