Wait for 3D Tiles loading when rendering offline

[kring]

When recording videos, we'd like to wait for 3D Tiles to completely finish loading before snapping each frame.

From @AlbanBERGERET-Epic:

Synchronisation mechanism for rendering a movie waiting for streaming completed:
You can use the sequencer as you did, but you'll have to produce the video using the new Movie Render Pipeline.
https://docs.unrealengine.com/en-US/Engine/Sequencer/Workflow/RenderAndExport/HighQualityMediaExport/index.html

When using this pipeline, you can define your own setting class by implementing UMoviePipelineSettings. If you do that in your own code it should automatically show up in the dropdown list in the UI.
There's a project setting which lets you specify which classes are added to new jobs by default too, so you can ensure your setting always gets added.
When that setting class is initialized, you can register to the engine begin tick event, and you can flush the streaming every frame before the engine ticks and tries to use the data.

[kring]

Implemented in #169.