Add motion vector support for animated surfaces

[DarioSamo]

Extends mesh instances that required custom vertex buffers to create two alternating buffers that are written to and binds them to use them as the previous vertex buffer when generating motion vectors.

Looking for some input on some of the implementation decisions.

[clayjohn]

Just a general comment from testing, before I look at the code. I can confirm that this makes the ghosting from using skeletons go away, so great work so far!

I did notice an issue that we may want to investigate further. If an animation is stopped (either because the user paused it or it reached the end and isn't set to loop) then the current implementation continues to report motion vectors from the last frame which leads to continuous jitter on that mesh.

To debug this I multipied the motion vectors by 50 in copy_to_fb.glsl

Animation stopped (no MVs visible, TAA resolves nicely):

Animation reaches end (MVs frozen in place, TAA not resolving:

I guess we need a way to flag for the renderer that the skeleton hasn't updated in this frame, in which case it doesn't even need to write out motion vectors.

[DarioSamo]

There's a few possible solutions to that.

Currently the one swapping the buffers is update_mesh_instances. If pausing the animation here makes that no longer update, then we could choose to validate the vertex array chosen based on the current frame number and the 'last change' stored in the surface itself (mesh_instance_surface_get_vertex_arrays_and_format most likely). The current implementation should handle that pretty gracefully if that's the only change we need to do.

[clayjohn]

Left a couple comments from code review

[DarioSamo]

Submitting for review, we might want some more extensive testing to spot any other issues if they arise.

[clayjohn]

Just the one nitpicky comment. Otherwise this looks great! Tested locally and it works well

[clayjohn]

Looks great! I tested locally and everything works.

While I don't anticipate any issues, I think the scope of changes is large enough that we shouldn't cherrypick this into 4.1

[akien-mga]

Thanks!