Some minor optimizations

[turbedi]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable) N/A

Description

1st commit:
Revert a stackalloc de-optimization from #1734 (it was probably forgotten in #1750) and add a comment

2nd commit:
Replace all Span<T>.Fill(default) calls with Span<T>.Clear() , as Span<T>.Clear() is more optimized than Span<T>.Fill(default)

[codecov]

Codecov Report

Merging #1828 (c22919d) into master (a39fe4c) will decrease coverage by 0%.
The diff coverage is 80%.

@@          Coverage Diff           @@
##           master   #1828   +/-   ##
======================================
- Coverage      87%     87%   -1%     
======================================
  Files         936     936           
  Lines       48191   48193    +2     
  Branches     6037    6037           
======================================
+ Hits        42098   42099    +1     
- Misses       5092    5094    +2     
+ Partials     1001    1000    -1     

Flag	Coverage Δ
unittests	87% <80%> (-1%)	⬇️

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

Impacted Files	Coverage Δ
...iff/Compression/Decompressors/T6TiffCompression.cs	83% <25%> (-2%)	⬇️
.../ImageSharp/Formats/Webp/Lossless/Vp8LHistogram.cs	97% <80%> (ø)
src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs	94% <100%> (ø)
...ats/Tiff/Compression/Compressors/TiffLzwEncoder.cs	100% <100%> (ø)
src/ImageSharp/Formats/Webp/Lossless/CostModel.cs	100% <100%> (ø)
...ageSharp/Formats/Webp/Lossless/HistogramEncoder.cs	96% <100%> (ø)
...c/ImageSharp/Formats/Webp/Lossless/HuffmanUtils.cs	96% <100%> (ø)
...rc/ImageSharp/Formats/Webp/Lossy/Vp8EncIterator.cs	100% <100%> (ø)
src/ImageSharp/Formats/Webp/Lossy/Vp8Encoder.cs	93% <100%> (ø)
...rc/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs	97% <0%> (+<1%)	⬆️

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[brianpopow]

Looks good, thanks.

[brianpopow]

The negative coverage report 87% (-1%) seems to be off.

[JimBobSquarePants]

The diff only has 80% coverage according to the report so seems about right.