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