BUG: skipped pixels in Aitoff-Hammer pixelization

[neutrinoceros]

Bug report

Bug summary

The pixelisation routine used for r-normal plots in spherical coordinates is slightly broken, and it seems that some pixels are sometimes skipped in the vicinity of the equator.

Code for reproduction

import numpy as np
import yt
from yt.units import g, cm

shape = (1, 33, 200)

data = {
    "density": np.ones(shape) * g / cm**3,
}

ds = yt.load_uniform_grid(
    data,
    shape,
    bbox=np.array([[0.1, 1], [0.7, np.pi - 0.7], [0, 2 * np.pi]]),
    geometry="spherical",
)

p = yt.SlicePlot(ds, "r", "density")

p.save("/tmp/res.png")

Actual outcome

In this example we get a series of "holes" in the image, all seemingly symmetrical with respect to the equator.
I note that this phenomenon is apparently very sensitive to the grid's resolution, I just selected the case for with I found the highest number of such artefacts, but I saw this happening at a smaller scale in production.

[neutrinoceros]

It quickly gets much worse as the resolution in colatitude is reduced.
this is with shape = (1, 9, 200)

and with shape = (1, 3, 200)

however it seems that this happens specifically if the number of cells in the colatitudinal direction is odd
, with shape = (1, 2, 200) it gets back to normal

[neutrinoceros]

It's probably easier to reason about if we only consider very small resolutions
shape = (1, 3, 6)