Add the ability to choose the filter method to use when encoding a png

[JimBobSquarePants]

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

Prior to this PR the png encoder always used adaptive filtering when choosing the filter method to use to provide optimum compression. This generally provides the smallest output but comes at a performance cost since the scanline has to be filtered several times before compression.

This PR adds the ability to choose each of the 5 filtering methods to use per scanline row, plus adaptive filtering which gives the developer great flexibility to encode pngs to their requirements.

[codecov]

Codecov Report

Merging #550 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #550      +/-   ##
=========================================
- Coverage   88.15%   88.1%   -0.06%     
=========================================
  Files         838     838              
  Lines       34948   34974      +26     
  Branches     2527    2529       +2     
=========================================
+ Hits        30810   30814       +4     
- Misses       3394    3398       +4     
- Partials      744     762      +18

Impacted Files	Coverage Δ
...ts/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs	97.67% <100%> (+0.41%)	⬆️
src/ImageSharp/Formats/Png/PngEncoder.cs	100% <100%> (ø)	⬆️
src/ImageSharp/Formats/Png/PngEncoderCore.cs	90.56% <100%> (+0.56%)	⬆️
src/ImageSharp/Common/Helpers/ImageMaths.cs	77.41% <0%> (-6.46%)	⬇️
...sing/Dithering/ErrorDiffusion/ErrorDiffuserBase.cs	93.1% <0%> (-3.45%)	⬇️
...Formats/Jpeg/GolangPort/Components/Decoder/Bits.cs	69.69% <0%> (-3.04%)	⬇️
...arp.Tests/TestUtilities/TestEnvironment.Formats.cs	87.87% <0%> (-3.04%)	⬇️
.../ImageSharp.Tests/TestUtilities/TestEnvironment.cs	82.5% <0%> (-2.5%)	⬇️
...c/ImageSharp/Formats/Png/Zlib/ZlibDeflateStream.cs	61.4% <0%> (-1.76%)	⬇️
...TestUtilities/Attributes/ImageDataAttributeBase.cs	83.33% <0%> (-1.67%)	⬇️
... and 18 more

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[dlemstra]

Nice addition

[tocsoft]

👍 looking good

[antonfirsov]

LGTM