Consider merging pure virtual class interfaces with base implementation

[osrf-migration]

Original report (archived issue) by Ian Chen (Bitbucket: Ian Chen, GitHub: 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

[osrf-migration]

Original comment by Nate Koenig (Bitbucket: Nathan Koenig).

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• Edited issue description

[osrf-migration]

Original comment by Nate Koenig (Bitbucket: Nathan Koenig).

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• Edited issue description