Win32/ARM64: let GraphicsAdapterSelectionCallback see unfiltered adapters and warn on Adreno WARP fallback

[nickna]

What does the pull request do?

On Windows-on-ARM (ARM64), AngleWin32EglDisplay applies a blanket blocklist that removes every Adreno adapter from the DXGI adapter list before selecting a Direct3D 11 device. This PR keeps the conservative default (don't render on Adreno) but stops hiding those adapters from Win32PlatformOptions.GraphicsAdapterSelectionCallback, and logs a warning when the blocklist forces a software fallback.

Both changes were agreed to by @maxkatz6 in #21689 (comment) (the scoped-blocklist and Vulkan-by-default proposals from the issue are intentionally not included).

What is the current behavior?

On any ARM64 device, the blocklist disposes and removes all adapters whose description contains adreno. The GraphicsAdapterSelectionCallback only ever receives the already-filtered list, so an application cannot opt back into hardware acceleration on a healthy Adreno GPU. Nothing is logged, so the resulting fall back to the Microsoft Basic Render Driver (WARP) — and its CPU-usage cost — is silent.

What is the updated/expected behavior with this PR?

• When a GraphicsAdapterSelectionCallback is supplied, it receives the full, unfiltered adapter list and its choice is used as-is. This gives applications a way to select an Adreno adapter deliberately while keeping the default ANGLE/D3D backend.
• When no callback is supplied, the Adreno blocklist behaviour is preserved: the default selection moves to the first non-Adreno adapter, and a Warning-level message is logged explaining the CPU impact and the available workarounds (GraphicsAdapterSelectionCallback or Win32RenderingMode.Vulkan).

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

The adapter list is no longer mutated. The blocklist now only computes the default index (first non-Adreno adapter) and is skipped entirely when a callback is present. As a side effect this removes a latent IndexOutOfRangeException: the previous code could empty the list when every enumerated adapter was Adreno.

Checklist

• Added unit tests (if possible)? — the code path depends on live DXGI adapter enumeration and ARM64 hardware, so it isn't covered by the existing unit-test harness.
• Added XML documentation to any related classes? — behaviour is documented via an inline comment; no public API changed.
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Behavioural change (no API change): on ARM64, a GraphicsAdapterSelectionCallback now receives Adreno adapters that were previously filtered out, so any index-based logic inside a callback will see a longer list.

Fixed issues

Refs #21689

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[nickna]

@cla-avalonia agree

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[MrJul]

LGTM!