Support antialiased lines in *_n reductions

[ianthomas23]

This adds support for antialiased lines in *_n reductions such as max_n and first_n. They return reasonable results, but these are not necessarily consistent with the non-*_n reductions yet. Support for antialiased where reductions will follow this, and then a full review of whether the results are consistent for all of the different line types supported (axis 0, axis 1, etc).

Example of output using this script

import datashader as ds
import numpy as np
import pandas as pd

df = pd.DataFrame(dict(
    ystart=[0.2, 0, 1, 0.8],
    yend=[0.2, 1, 0, 0.8],
    value=[2, 4, 1, 3],
))
kwargs = dict(source=df, x=np.asarray([0, 1]), y=["ystart", "yend"], axis=1, line_width=15)

reductions = dict(
    first_n=ds.first_n("value", n=2),
    last_n=ds.last_n("value", n=2),
    min_n=ds.min_n("value", n=2),
    max_n=ds.max_n("value", n=2),
    min_n_row_index=ds._min_n_row_index(n=2),
    max_n_row_index=ds._max_n_row_index(n=2),
)

cvs = ds.Canvas(plot_width=200, plot_height=150, x_range=(-0.1, 1.1), y_range=(-0.1, 1.1))
for name, reduction in reductions.items():
    agg = cvs.line(agg=reduction, **kwargs)
    span = (np.nanmin(agg), np.nanmax(agg))
    for k in range(agg.shape[2]):
        im = ds.transfer_functions.shade(agg[:, :, k], how="linear", span=span)
        ds.utils.export_image(im, f"temp_{name}_{k}", background="white")

first_n showing [:, :, 0] then [:, :, 1]:

last_n:

min_n:

max_n:

_min_n_row_index:

_max_n_row_index:

[codecov]

Codecov Report

Merging #1262 (1f91111) into main (0b71920) will decrease coverage by 0.19%.
The diff coverage is 67.36%.

@@            Coverage Diff             @@
##             main    #1262      +/-   ##
==========================================
- Coverage   83.62%   83.43%   -0.19%     
==========================================
  Files          35       35              
  Lines        8938     9069     +131     
==========================================
+ Hits         7474     7567      +93     
- Misses       1464     1502      +38     

Files Changed	Coverage Δ
datashader/core.py	88.25% <ø> (ø)
datashader/utils.py	76.89% <28.84%> (-5.82%)	⬇️
datashader/compiler.py	90.03% <83.33%> (-0.21%)	⬇️
datashader/reductions.py	80.10% <90.14%> (+0.94%)	⬆️
datashader/antialias.py	96.87% <100.00%> (+0.10%)	⬆️

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