Respecting fileHeader Offset, skipping bytes if necessary

[brianpopow]

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

The file header offset is ignored, which indicates where the image data starts. This can cause incorrectly decoded images.

This PR changes this and unused bytes between the palette and the image will be skipped.

[codecov]

Codecov Report

Merging #819 into master will increase coverage by 0.08%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #819      +/-   ##
==========================================
+ Coverage   88.65%   88.74%   +0.08%     
==========================================
  Files        1015     1015              
  Lines       44120    44130      +10     
  Branches     3178     3180       +2     
==========================================
+ Hits        39116    39164      +48     
+ Misses       4241     4237       -4     
+ Partials      763      729      -34

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...ts/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs	98.92% <100%> (+0.06%)	⬆️
src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs	83.21% <60%> (+0.19%)	⬆️
...files/ICC/DataWriter/IccDataWriter.TagDataEntry.cs	77.14% <0%> (ø)	⬆️
src/ImageSharp/Processing/ResizeHelper.cs	42.42% <0%> (ø)	⬆️
...ils/ReferenceImplementations.StandardIntegerDCT.cs	93.68% <0%> (ø)	⬆️
src/ImageSharp/Formats/Png/PngEncoderCore.cs	93.51% <0%> (+0.27%)	⬆️
...files/ICC/DataReader/IccDataReader.TagDataEntry.cs	85.91% <0%> (+0.28%)	⬆️
...harp.Tests/Drawing/FillLinearGradientBrushTests.cs	99.5% <0%> (+0.49%)	⬆️
... and 15 more

---

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[codecov]

Codecov Report

Merging #819 into master will decrease coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff            @@
##           master    #819      +/-   ##
=========================================
- Coverage    88.8%   88.8%   -0.01%     
=========================================
  Files        1015    1015              
  Lines       44138   44148      +10     
  Branches     3178    3180       +2     
=========================================
+ Hits        39198   39206       +8     
- Misses       4211    4212       +1     
- Partials      729     730       +1

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...ts/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs	98.92% <100%> (+0.06%)	⬆️
src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs	83.21% <60%> (-0.27%)	⬇️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[JimBobSquarePants]

@brianpopow Again amazing work, you rock! 👍