Add support for encoding 1 bit per pixel bitmaps

[brianpopow]

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Description

Add support for encoding 1 bit per pixel bitmaps.

@JimBobSquarePants: The bitmap encoder uses Octree Quantizer as a default. This quantizer does seem to have problems with just 2 colors. Im not so familiar with the Octree Quantizer. Is not possible to quantize with just to colors or is it maybe an issue we should look into? All indices seem to be 0.
If i change it to Wu quantizer it works with two colors.

[codecov]

Codecov Report

Merging #1623 (33192d3) into master (6fa903c) will increase coverage by 0.00%.
The diff coverage is 88.57%.

@@           Coverage Diff           @@
##           master    #1623   +/-   ##
=======================================
  Coverage   83.67%   83.67%           
=======================================
  Files         749      749           
  Lines       33082    33111   +29     
  Branches     3707     3714    +7     
=======================================
+ Hits        27682    27707   +25     
- Misses       4681     4682    +1     
- Partials      719      722    +3     

Flag	Coverage Δ
unittests	83.67% <88.57%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/ImageSharp/Formats/Bmp/BmpEncoder.cs	100.00% <ø> (ø)
src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs	93.83% <88.23%> (-1.08%)	⬇️
src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs	94.21% <100.00%> (-0.06%)	⬇️

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[JimBobSquarePants]

This quantizer does seem to have problems with just 2 colors. Im not so familiar with the Octree Quantizer. Is not possible to quantize with just to colors or is it maybe an issue we should look into? All indices seem to be 0.

@brianpopow Yep there's issues with how the Octree Quantizer currently distributes and reduces colors. (In addition to rampant memory usage issues)

It does look like we should be able to do a fairly quick port based on the code here to a version that should allow accurate reduction.
#1350 (comment)

It's something I keep meaning to take the time to attempt.

[brianpopow]

@brianpopow Yep there's issues with how the Octree Quantizer currently distributes and reduces colors. (In addition to rampant memory usage issues)

@JimBobSquarePants: Should I switch the default quantizer then to the Wu-Quantizer?

[JimBobSquarePants]

@JimBobSquarePants: Should I switch the default quantizer then to the Wu-Quantizer?

For bitmap so we can get this PR in then yeah. I'll likely move back after depending on the results of profiling both once things have been fixed.

[JimBobSquarePants]

Awesome work. I can't believe we have all the bit depth support now! 😄

[rickbrew]

It does look like we should be able to do a fairly quick port based on the code here to a version that should allow accurate reduction.
#1350 (comment)

I can confirm that my new Octree code correctly quantizes down to 1-bit depth. Paint.NET uses this in production today with the latest 4.2.16 release.

[JimBobSquarePants]

@rickbrew Awesome, that's great to hear, thanks!