Shadows and appearances improvements

@Keboo がすでに取り組んでいます。

2021年7月16日 から。

Hello to all!
I want to suggest some performance improvements for animations for MaterialDesignInXamlToolkit library and resolve some issues, I faced with.

So, the first one: don't use BitmapCache for Shadows and for animations. I suggest to use FastShadow (https://stackoverflow.com/questions/961875/wpf-animation-ui-features-performance-and-benchmarking) with the same appearance. Why don't use BitmapCache (and related ShadowAssist with default set for CacheMode)? BitmapCache invokes some problems on WPF, for example when user has multiple windows. And if those shadows are not static (for example when animate them in DrawerHost or other controls) I explained it in #2260 and https://stackoverflow.com/questions/66471514/wpf-rendering-issue-on-any-machine-related-to-bitmapcache. But if you want to test on your machine, please test this repo - https://github.com/Erapchu/GhostWindows
And please, just use one ShadowAssist instance instead of directly BitmapCache when coding in XAML.
Additionally, may be resolve issue with graphic artifacts? I remember one artifact related to #2125 issue.

This is just suggestions, and if maintainers don't want to rewrite some code, I'll understand it. Because it's may be hard work to rewrite controls and handle all of situations.