Improve support for NdOverlay and Overlay in downsample1d

[hoxbro]

Before the following code will always return the non-downsampled ndoverlay/overlay.

        if len(element) <= self.p.width:
            return element

[codecov-commenter]

Codecov Report

Merging #5856 (76a17bb) into main (ee01948) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5856      +/-   ##
==========================================
+ Coverage   88.24%   88.30%   +0.05%     
==========================================
  Files         309      310       +1     
  Lines       63921    63944      +23     
==========================================
+ Hits        56408    56466      +58     
+ Misses       7513     7478      -35     

Flag	Coverage Δ
ui-tests	23.38% <30.43%> (+0.02%)	⬆️

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Files Changed	Coverage Δ
holoviews/operation/downsample.py	84.90% <100.00%> (+84.90%)	⬆️
holoviews/tests/operation/test_downsample.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[philippjfr]

Looks good!