Optimize TryGetSinglePixelSpan

[JimBobSquarePants]

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 matches 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)

Description

Saw someone complaining on Discord about this so fixed it.

[codecov]

Codecov Report

Merging #1407 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1407   +/-   ##
=======================================
  Coverage   82.99%   82.99%           
=======================================
  Files         692      692           
  Lines       31189    31189           
  Branches     3578     3578           
=======================================
  Hits        25884    25884           
  Misses       4582     4582           
  Partials      723      723           

Flag	Coverage Δ
#unittests	82.99% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/ImageSharp/Image{TPixel}.cs	92.68% <100.00%> (ø)

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[Sergio0694]

LGTM! Great to see that LINQ path gone 😄🚀