Optimize rendering performance by reduce Premultiplied AlphaMode

[lindexi]

What does the pull request do?

See #20643

Testing the RenderDemo in my device:

• Screen: 3840x2160 (4K) + 100% DPI
• CPU: i5-12450H
• GPU: - (Integrated graphics card)

Test step:

1. Open the TaskManager to find the GPU usage
2. Switching the TransparencyLevelHint by the menu item in RenderDemo:
   

GPU usage of DWM:

Win32CompositionMode	Before	After
WinUIComposition	45%	15%
DirectComposition	35%	15%

What is the current behavior?

Currently, regardless of whether a window requires transparency, the rendering backend on Windows is configured to use the Premultiplied AlphaMode. This forces the Desktop Window Manager (DWM) to handle alpha compositing even for opaque windows, resulting in unnecessary GPU resource overhead for the DWM. See #20643

What is the updated/expected behavior with this PR?

With the proposed modification, render backend will first determine if a window is opaque by WindowTransparencyLevel as @maxkatz6 recommended (see #20643 (comment) ). If so, it will set the AlphaMode in the rendering backend to "Ignore". This adjustment enables the GPU to conserve substantial resources when processing opaque windows.

Notably, this change will lead to a significant reduction in GPU resource usage on 4K-resolution devices without discrete graphics cards, especially when full-screen opaque windows are active.

How can I apply this optimization? Just settings the window.TransparencyLevelHint = [WindowTransparencyLevel.None];

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/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

Fixes #20643

[avaloniaui-bot]

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[avaloniaui-bot]

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[avaloniaui-bot]

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[avaloniaui-bot]

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[avaloniaui-bot]

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[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0063050-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]