Fixed compositor nullreference in WinUI 3

[Arlodotexe]

This PR closes #235, #233

PR Summary

Introduces a hotfix for a regression in the WinUI 3 package where usage of the compositor in the PipelineBuilder resulted in a NullReferenceException, caused by usage of Window.Current.Compositor instead of the WinUI 3 equivalent.

Still unable to launch the Wasdk sample app head for both the single-component Media solution and the gallery due to #212 and #169. These changes were tested by producing NuGet packages and testing in a blank Wasdk app.

PR Checklist

Please check if your PR fulfills the following requirements:

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• Tested code with current supported SDKs
• New component
  • Documentation has been added
  • Sample in sample app has been added
  • Analyzers are passing for documentation and samples
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (if applicable)
• Header has been added to all new source files
• Contains NO breaking changes

[michael-hawker]

LGTM