Use latest Avalonia 0.9.1

[worldbeater]

Updated the projects to use latest Avalonia and lifetimes API. Also added a new SourceUriRaw attached property to allow folks to write somewhat like the following directly in XAML, without bindings:

<Image Stretch="UniformToFill" 
       gif:GifImage.SourceUriRaw="resm:AvaloniaGif.Demo.Images.loader.gif"/>

Assuming the loader.gif is located inside the same assembly as EmbeddedResource. This can be sometimes useful, e.g. when the view models live in a separate project and know nothing about the currently used GUI framework

[jmacato]

Thanks for the PR @worldbeater! :D Just need to correct some nits :) I haven't been able to maintain this repo, and we still need rendering changes in Avalonia as described by @kekekeks in AvaloniaUI/Avalonia#3014 to bringout the best performance out of this library.

[worldbeater]

Thanks for taking the time to review this @jmacato! Applied the suggested tweak and added a demo.gif to README.md to give folks more context about what they are looking at. Decided to add an animated circle progress bar to a sample app I'm working on in my spare time, and your implementation just worked 👍 (haven't noticed any performance issues yet btw)

[jmacato]

@worldbeater Awesome! :D There's still some issues that i want to fix when i get some free time, @Kermalis 's issue on #2 is a high-priority and im still struggling on why that happens.

[jmacato]

LGTM!