feat: software-mode external texture support for D3D11-hooked apps

[guide-nhant]

No description provided.

[Copilot]

Pull request overview

Adds software-renderer support for external pixel-buffer textures so the Texture widget can display plugin-provided frames even when the engine falls back to software rasterization (e.g., when ANGLE/D3D11 isn’t available on Windows).

Changes:

• Extends the embedder API (FlutterSoftwareRendererConfig) with an optional software external-texture callback returning a CPU pixel buffer.
• Wires the Windows software renderer config to ask the Windows texture registrar for pixel buffers in software mode.
• Introduces a new software external texture implementation (EmbedderExternalTextureSoftware) and integrates it into the external texture resolver.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
engine/src/flutter/shell/platform/windows/flutter_windows_texture_registrar.h	Adds software populate API and stores pixel-buffer callbacks for software rendering.
engine/src/flutter/shell/platform/windows/flutter_windows_texture_registrar.cc	Registers pixel-buffer callbacks for software path; implements PopulateTextureSoftware; updates unregister behavior.
engine/src/flutter/shell/platform/windows/flutter_windows_engine.cc	Sets external_texture_frame_callback in the software renderer config for Windows.
engine/src/flutter/shell/platform/embedder/embedder.h	Adds FlutterSoftwarePixelBuffer + software external texture callback fields to software renderer config.
engine/src/flutter/shell/platform/embedder/embedder.cc	Creates an external texture resolver backed by the new software external texture callback when in software mode.
engine/src/flutter/shell/platform/embedder/embedder_external_texture_software.h	Declares the software external texture implementation.
engine/src/flutter/shell/platform/embedder/embedder_external_texture_software.cc	Implements software external texture rendering via raster SkImage creation from CPU pixels.
engine/src/flutter/shell/platform/embedder/embedder_external_texture_resolver.h	Adds software external texture callback plumbing to the resolver.
engine/src/flutter/shell/platform/embedder/embedder_external_texture_resolver.cc	Resolves textures using the software implementation when configured.
engine/src/flutter/shell/platform/embedder/BUILD.gn	Adds the new software external texture sources to the embedder build.

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[guide-nhant]

Release build here
https://github.com/guide-inc-org/guide-flutter/releases/tag/3.29.3_v3_external_texture_support