Allow pixel_ratio to go below 1

[jbednar]

For some reason datashade/rasterize pixel_ratio was constrained to increasing the resolution, but it should also be useful for reducing the resolution:

import holoviews as hv, numpy as np
from holoviews.operation.datashader import datashade, rasterize, dynspread
hv.extension("bokeh")

p = hv.Points(np.random.multivariate_normal((0,0), [[0.1, 0.1], [0.1, 1.0]], (300,)))
dynspread(rasterize(p, pixel_ratio=2)) + dynspread(rasterize(p, pixel_ratio=0.25))

Now checks that the ratio is above zero, rather than 1 or larger:

[codecov-commenter]

Codecov Report

Merging #5813 (65d33ed) into main (003be41) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5813   +/-   ##
=======================================
  Coverage   88.17%   88.17%           
=======================================
  Files         307      307           
  Lines       62916    62916           
=======================================
  Hits        55479    55479           
  Misses       7437     7437           

Flag	Coverage Δ
ui-tests	22.40% <100.00%> (ø)

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Impacted Files	Coverage Δ
holoviews/operation/resample.py	90.26% <100.00%> (ø)

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[github-actions]

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