Use Span in GetHTreeGroupForPos to avoid allocations

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

This PR will remove an unnecessary call to ToArray() and use Span instead.

Before:

After:

Example is for earth_lossless.webp

[codecov]

Codecov Report

Merging #1793 (e168ae6) into master (d3f3eec) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master    #1793   +/-   ##
=======================================
  Coverage   87.03%   87.04%           
=======================================
  Files         936      936           
  Lines       47649    47649           
  Branches     6015     6015           
=======================================
+ Hits        41472    41474    +2     
+ Misses       5182     5181    -1     
+ Partials      995      994    -1     

Flag	Coverage Δ
unittests	87.04% <66.66%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...Sharp/Formats/Webp/Lossless/WebpLosslessDecoder.cs	94.67% <66.66%> (ø)
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	89.01% <0.00%> (+0.19%)	⬆️
...ImageSharp/Formats/Webp/Lossless/Vp8LBitEntropy.cs	100.00% <0.00%> (+1.19%)	⬆️

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[antonfirsov]

LGTM, seems trivial.