Add a sprite outline shader

## What problem does this solve or what need does it fill?

Shading the outlines of objects is a very common effect, either globally or when objects are selected.

This is much nicer to be able to do dynamically / procedurally than as part of your asset creation, because then the effect can be varied and different levels of outlining can be tweaked without having to redo assets.

## What solution would you like?

We ship something configurable in the engine itself.

## What alternative(s) have you considered?

We create a detailed high quality example teaching users how to do this.

The issue names no files, tests, or entry points. Start by determining whether the project should ship configurable engine support or provide a detailed example, then define the required outline behavior and configuration surface. Done means one of those approaches is implemented and usable for dynamic or procedural sprite outlines.

Written by the indexing model from the issue text.