BUG: fix a bug in pixelize_cylinder where buffer shape was read backwards

[neutrinoceros]

PR Summary

Welp, that was easy.
Fix #3794

Just to make it flagrant I'll make an even more drastic version of the script therein

import numpy as np
import yt

shape = (32, 16, 1)
a = np.linspace(1, 100, 16)
b = np.ones((32, 16))
c = np.reshape(a * b, shape)
data = {("gas", "density"): c}

ds = yt.load_uniform_grid(
    data,
    shape,
    bbox=np.array([[0.0, 5.0], [np.pi / 2 - 0.2, np.pi / 2 + 0.2], [-0.1, +0.1]]),
    geometry="spherical",
)

p = yt.SlicePlot(ds, "phi", "density")
p.save("/tmp/buff_default.png")
p.set_buff_size((10, 800))
p.save("/tmp/buff_10_800.png")

p.set_buff_size((10, 10))
p.save("/tmp/buff_10_10.png")

[neutrinoceros]

I checked that this also fixes a similar issue in plotting "z" normal slice plots in cylindrical geometry.

[matthewturk]

This looks like it will fix a bunch of stuff.