Added temporal reprojection to Volumetric Fog

[reduz]

-It's an option, just enable it
-Just works, don't have to do anything else.

volufog_repro.mp4

[clayjohn]

Looks good for now!

This will need to be updated once we add motion vectors as this sort of reprojection really only works for static objects. Dynamic objects are going to create a lot of ghosting artifacts.

[reduz]

@clayjohn oh dynamic geometry/motion vectors makes zero difference on this, only dynamic light changes do. Since all that is being re-projected is the frustum.

[clayjohn]

But that's why you'll end up with ghosting. Moving a dynamic object across the screen won't properly invalidate the samples from the previous frame.

The problem is reduced because you are reprojection the entire frustum instead of just the screen-space samples, but I'm sure there will be ghosting.

At any rate. It's fine as is!

[reduz]

@clayjohn oh but then again remember the volumetric fog contains no geometry or solid object information. For all objects care its the same as a regular exponential fog, and generally information is so low frequency that i doubt it will cause issues with motion vectors.