Blender Git Loki

Refactor object engine data storage

Main idea is to make specific engine types be a subclass of generic
ObjectEngineData structure.

This required following changes:

- Have extra size argument to engine data allocation function.

Not sure whether there is less error-prone way of doing this.

- Add init() callback to engine data allocation function.

Additionally, added some extra checks to Eevee's engine data getters, so we do
not silently cast lamp data to lightprobe data.

Reviewers: dfelinto, fclem

Differential Revision: https://developer.blender.org/D3027