Add a blendMode to BackdropFilter layers to enable developers to control blending on save layers (#19631)

This avoids the awkward capitalisation 'E.g.' but more importantly
triggers a build with the latest recipe update, which includes Windows
UWP build products.

* Roll CanvasKit to 0.25.1

* Update SkParagraph tests

The FlutterComputePlatformResolvedLocaleCallback does not pass the
standard user_data baton. This was an unintentional omission in the
original patch.

Add documentation to that effect and link to the bug report:
https://github.com/flutter/flutter/issues/79826

FlutterDesktopEngineCreate is part of our C API. We were using a C++
reference type instead of a C-compatible pointer type.

This is a breaking change to anyone calling this directly; we believe
this should affect few people because the Windows template only uses the
`FlutterEngine` wrapper in
`shell/platform/windows/client_wrapper/include/flutter/flutter_engine.h`.

Fixes https://github.com/flutter/flutter/issues/75465