Implement bitmap transcoding for most supported PixelFormats

[Gillibald]

What does the pull request do?

This PR introduces a new interface (IPixelFormatWriter) for writing pixels of different formats.

The IPixelFormatWriter is then used in the new PixelFormatTranscoder class that allows the transcoding of pixels into a different format.

The previous helper method to transcode a bitmap was replaced by the new component.

New API

void Bitmap.CopyPixels(ILockedFramebuffer buffer, AlphaFormat alphaFormat)

What is the current behavior?

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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