stackalloc header buffer in InternalDetectFormat

[antonfirsov]

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

The buffer allocation/disposal has shown up as a small but visible bottleneck in PNG decoder profiling results. Headers are always very small, we should stackalloc the buffer for them.

[codecov]

Codecov Report

Merging #1812 (af90336) into master (a06011a) will increase coverage by 0.14%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1812      +/-   ##
==========================================
+ Coverage   87.19%   87.33%   +0.14%     
==========================================
  Files         936      936              
  Lines       47920    47922       +2     
  Branches     6012     6016       +4     
==========================================
+ Hits        41783    41852      +69     
+ Misses       5144     5078      -66     
+ Partials      993      992       -1     

Flag	Coverage Δ
unittests	87.33% <85.71%> (+0.14%)	⬆️

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

Impacted Files	Coverage Δ
src/ImageSharp/Image.Decode.cs	86.76% <85.71%> (-1.12%)	⬇️
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	98.10% <0.00%> (+5.29%)	⬆️
.../ImageSharp/Formats/Webp/Lossless/LosslessUtils.cs	97.61% <0.00%> (+6.80%)	⬆️

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[JimBobSquarePants]

Nice easy one to review this 👍