Render accumulation (allow custom background blending)

[jkulhanek]

This PR enabled accumulation to be rendered and returned.

[ladzin]

I tried this fork and it worked great. But I made a mistake: I assumed that gradients with respect to "accumulation" would flow correctly, but that was not the case since backward wasn't implemented -- so I got, silently, the wrong gradients. Would it be helpful to add an error/warning or implement the backward pass?

[aixiaodewugege]

Thanks for your brilliant work. Does the original version not support a custom background? Is there any document explain it?

[MatteoMarengo]

Hello !
I had a question, does this fork allows us to start with a customized image and then train the 3DGS with this image as the background ?

[HaedamOh]

I think this PR does not implement backward pass. I can forward "accumulation" but I could see no grad in backward pass.