Use bulk pixel conversion for some tiff color decoders

[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 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

As the title says: Use bulk pixel conversion for some tiff color decoders
This should be a bit faster the converting pixel by pixel.

[antonfirsov]

LGTM 👍

[codecov]

Codecov Report

Merging #1726 (01202b6) into master (7da6c33) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1726   +/-   ##
=======================================
  Coverage   84.36%   84.37%           
=======================================
  Files         822      822           
  Lines       36023    36033   +10     
  Branches     4204     4200    -4     
=======================================
+ Hits        30392    30404   +12     
+ Misses       4816     4814    -2     
  Partials      815      815           

Flag	Coverage Δ
unittests	84.37% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...icInterpretation/BlackIsZero16TiffColor{TPixel}.cs	100.00% <100.00%> (ø)
...ricInterpretation/BlackIsZero8TiffColor{TPixel}.cs	100.00% <100.00%> (ø)
...metricInterpretation/Rgb161616TiffColor{TPixel}.cs	100.00% <100.00%> (ø)
...otometricInterpretation/Rgb888TiffColor{TPixel}.cs	100.00% <100.00%> (ø)
...cInterpretation/TiffColorDecoderFactory{TPixel}.cs	75.00% <100.00%> (ø)
src/ImageSharp/Formats/Tiff/TiffDecoderCore.cs	88.70% <100.00%> (ø)
.../PixelFormats/PixelOperations{TPixel}.Generated.cs	99.16% <0.00%> (+0.83%)	⬆️

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