remove SavePixelData() apis in favour of GetPixelSpan()

[tocsoft]

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Description

removed all the SavePixelData() apis (as they can cause confusion people see a save method with a byte array and assume its an encoded image they can pass into something else and its not it was just a raw dump of our internal buffer).

To get a copy of the image buffer now you should instead add the using SixLabors.ImageSharp.Advanced; which will expose our Span apis which can be used to access the raw pixel buffer for any inter-op scenarios.

resolves #623

[codecov]

Codecov Report

Merging #628 into master will decrease coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
- Coverage   88.63%   88.55%   -0.08%     
==========================================
  Files         884      884              
  Lines       37068    37032      -36     
  Branches     2670     2670              
==========================================
- Hits        32855    32794      -61     
- Misses       3418     3421       +3     
- Partials      795      817      +22

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/Image/ImageSaveTests.cs	100% <ø> (ø)	⬆️
src/ImageSharp/ImageExtensions.cs	68.42% <ø> (-0.97%)	⬇️
...ests/Processing/Transforms/AffineTransformTests.cs	98.51% <100%> (-0.02%)	⬇️
...Processing/Transforms/ProjectiveTransformHelper.cs	86.11% <0%> (-11.12%)	⬇️
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%)	⬇️
...sts/Formats/Jpg/DoubleBufferedStreamReaderTests.cs	97.33% <0%> (-2.67%)	⬇️
.../ImageSharp.Tests/TestUtilities/TestEnvironment.cs	81.25% <0%> (-2.09%)	⬇️
...c/ImageSharp/Formats/Png/Zlib/ZlibDeflateStream.cs	61.4% <0%> (-1.76%)	⬇️
... and 17 more

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