Consider merging pure virtual class interfaces with base implementation

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-rendering/issues/22)) by Ian Chen (Bitbucket: [Ian Chen](https://bitbucket.org/%7Beaa6fca5-6deb-43f6-907f-971c144735dd%7D/), GitHub: [iche033](https://github.com/iche033)).**

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Instead of having pure virtual interfaces in `include/ignition/rendering`, we can merge the Base* implementations from `include/ignition/rendering/base/` in to `src`.

It's a trade off between cleaner separation of code and improving code maintainability and extensibility:

Main advantages:

* reduces the number of files to maintain
* easier to add/update/remove features or functions in classes

Templated Base classes also them difficult to add private implementations using the pimpl pattern