Refactor library so it's harder to break ABI

Adding new features to a released version of `ign-rendering` is complicated because of its current inheritance structure. All (most?) [user-facing classes](https://github.com/ignitionrobotics/ign-rendering/tree/main/include/ignition/rendering) are pure-virtual, and adding new pure virtual functions breaks ABI.

We should look into refactoring `ign-rendering` in a way that it's easier to add new APIs without breaking ABI, so that we don't require major version bumps for new features.

Start by reviewing the user-facing classes under include/ignition/rendering and their current inheritance structure. Trace how adding a pure-virtual function affects released ABI and identify the scope of a refactoring that permits new APIs without major version bumps. Done requires an agreed design and implementation plan; the issue provides no specific tests or acceptance criteria.

Written by the indexing model from the issue text.