add argument to provide a background depth when using RGB+D

[martinResearch]

When using RGB+D for rendering the depth, the depth in region or the image where there are not gaussians is zero.
This seems to be a somewhat arbitrary choice and in some cases we would like to use a positive constant value instead.
We can do that as a post-process using the rendered depth and the alpha value using
rendered_depth + (1-alpha) * background_depth, but it would make things clearer if the gsplat rendering function had a background_depth argument that defaults to 0 and that would allow the user to provide a different background depth.
To implement this we would modify the line here

gsplat/gsplat/rendering.py

Line 719 in 7951619

[backgrounds, torch.zeros(C, 1, device=backgrounds.device)], dim=-1

by [backgrounds, background_depth * torch.ones(C, 1, device=backgrounds.device)], dim=-1

[Jai2500]

Can I take this up? The change seems simple enough and would be my first contribution.
However, I suggest using
torch.full((C, 1), background_depth, device=backgrounds.device, dtype=backgrounds.dtype)
as that should remove the redundant filling with 1 and subsequent multiplication.