RenderEngineThreaded takes a function as an argument to create

In order to simplify testing and implementation of threaded RE,
RenderEngineThreaded::create can takes a std::function argument to
call to create the RE instance. RenderEngineThreaded::threadMain
then call the function.
- RenderEngine::create() uses it to create the production instance.
- The test uses it to create the mock instance.

Test: Unit tests
Change-Id: Ic1af134979d4449c34d9ad86d07673367eb16896