Advertise PreMultipled rather than PostMultiplied alpha for metal backend

[nicoburns]

Connections
Fixes #9896

Description
The Metal backend is advertising the wrong AlphaMode.

• This issues was first noticed in Support Configurable Composite Alpha Modes, Transparency, & Renderer Options DioxusLabs/anyrender#70 (comment)
• I have been unable to find official documentation confirming this (not entirely unsurprising for Apple)
• However, I had AI create a test case (can be viewed/run here: [DO NOT MERGE] Example to probe expected alpha mode on macOS/metal #9897) which tests the output by actually creating a transparent window (with an opaque black window positioned underneath), and I am personally convinced that this demonstrates that compositing on macOS is actually PreMultiplied (although my expertise in graphics is limited). I would encourage wgpu maintainers to review/run this example themselves and convince themselves of the same before merging this PR.

Testing
That is the correct alpha mode has been manually tested using the reproduction in #9897

Squash or Rebase?

Either - there is only one commit.

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.