Epic: introduce pixel-agnostic API across the whole library #907

I'm opening this to track the API changes I'm planning to introduce in multiple PR-s:

• Adding the non-generic Image base class, refactor and publish processors without color parameters. (Introduce a non-generic Image base class #904)
• Introduce pixel independent color type, refactor and publish processors having color values as parameters.
• Make ImageSharp.Drawing API non-generic.
• Do a sanity check on the resulting API set. Drop (or consider dropping) generic classes made obsolete by the new API (ColorBuilder<T>, NamedColors<T> and ImageProcessingContext<T>)
• Define (or at least considerdefining) a non-generic variant of ImageFrameCollection
• Update SixLabors/Samples (also taking other aspects into account)
• Anaylize impact on AOT, extend AotCompilerTools. EDIT: Won't do it for RC1. Moved to separate issue: Maintain Xamarin iOS support by keeping the library "AOT-proof" #946.

After finishing the changes, I'm planning to publish a blog post to explain the new API and help existing users with a migration guide.