Refactor IDisposable pattern

[JimBobSquarePants]

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Description

Touches #967

• Refactor of IDisposable pattern for images, frames, and processors to work better with inheritance and ensure finalizer suppression.
• Image<TPixel> is no longer sealed as that prevents extending for potential types like IntegralImage<TPixel>. ImageFrame<TPixel> is left sealed as inheritance should not be required.

[codecov]

Codecov Report

Merging #1009 into master will decrease coverage by 0.1%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1009      +/-   ##
==========================================
- Coverage   89.85%   89.74%   -0.11%     
==========================================
  Files        1098     1098              
  Lines       48823    48828       +5     
  Branches     3435     3435              
==========================================
- Hits        43870    43821      -49     
- Misses       4252     4254       +2     
- Partials      701      753      +52

Impacted Files	Coverage Δ
src/ImageSharp/ImageExtensions.cs	68.42% <ø> (-2.32%)	⬇️
...rp/Processing/Processors/ImageProcessor{TPixel}.cs	100% <100%> (+9.67%)	⬆️
...essing/Processors/CloningImageProcessor{TPixel}.cs	82.05% <100%> (+2.05%)	⬆️
src/ImageSharp/ImageFrame.cs	100% <100%> (ø)	⬆️
src/ImageSharp/Image.cs	93.75% <100%> (-0.54%)	⬇️
src/ImageSharp/ImageFrame{TPixel}.cs	86.41% <33.33%> (-1.09%)	⬇️
src/ImageSharp/Image{TPixel}.cs	89.65% <88.88%> (-0.35%)	⬇️
...xelFormats/PixelOperations{TPixel}.PixelBenders.cs	89.9% <0%> (-10.1%)	⬇️
...olorSpaces/Conversion/ColorSpaceConverter.Adapt.cs	60% <0%> (-8.58%)	⬇️
...Processing/Processors/Transforms/TransformUtils.cs	87.64% <0%> (-6.75%)	⬇️
... and 26 more

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[antonfirsov]

I disagree with opening up the class hierarchy, otherwise looks good. See also:
https://twitter.com/marcgravell/status/1172053321386987520

[JimBobSquarePants]

Gonna merge this without waiting for the build. I don't want to wait 40 minutes and we know it builds/passes tests.