Write exif profile with padding if needed

[brianpopow]

Prerequisites

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• 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 fixes #1796: adds a padding byte to the exif profile, if needed.

There still seems to be an issue with lossless. The issue was due to a mismatch of the alpha flag in VP8X and VP8L chunk.

[codecov]

Codecov Report

Merging #1798 (7f3c8ff) into master (b401937) will increase coverage by 0.05%.
The diff coverage is 89.65%.

@@            Coverage Diff             @@
##           master    #1798      +/-   ##
==========================================
+ Coverage   87.04%   87.09%   +0.05%     
==========================================
  Files         936      936              
  Lines       47631    47638       +7     
  Branches     6015     6017       +2     
==========================================
+ Hits        41462    41492      +30     
+ Misses       5177     5152      -25     
- Partials      992      994       +2     

Flag	Coverage Δ
unittests	87.09% <89.65%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Webp/WebpEncoderCore.cs	100.00% <ø> (ø)
...ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs	91.83% <82.35%> (+1.36%)	⬆️
.../ImageSharp/Formats/Webp/BitWriter/Vp8BitWriter.cs	95.80% <100.00%> (ø)
...ImageSharp/Formats/Webp/BitWriter/Vp8LBitWriter.cs	100.00% <100.00%> (ø)
...rc/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs	97.37% <100.00%> (ø)
src/ImageSharp/Formats/Webp/Lossy/Vp8Encoder.cs	93.60% <100.00%> (+0.01%)	⬆️
...ImageSharp/Formats/Webp/BitReader/Vp8LBitReader.cs	86.95% <0.00%> (-8.70%)	⬇️
...mageSharp/Formats/Webp/Lossless/NearLosslessEnc.cs	88.67% <0.00%> (-7.55%)	⬇️
.../ImageSharp/Formats/Webp/Lossless/LosslessUtils.cs	90.68% <0.00%> (+2.46%)	⬆️
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	92.67% <0.00%> (+3.66%)	⬆️

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[JimBobSquarePants]

LGTM 👍