Extend MirAL to offer a decent API for animations & custom compositing

Desire: to lower the amount of work community shells would have to do in adopting Mir (thus making Mir more desirable)

One feature identified as lacking in Mir is the existence of a simple graphics & animation engine, to do window animations and transitions. Third party shells typically customize these animations to have a unique look & feel.

The approach we did with Unity8, using a third party graphics engine (QtQuick), does not appeal to most shell authors, who instead just want an already-working basic implementation that they can customise.

The issue names no files, tests, or entry points. Begin by mapping MirAL's compositor and graphics APIs, then compare the Unity8/QtQuick approach with the needs of community shells. Done would require an agreed animation and custom-compositing API plus a basic implementation and validation plan.

Written by the indexing model from the issue text.