Try fusing depth data from non-keyframes into keyframes

[puzzlepaint]

Currently, non-keyframes almost do not contribute to the final result (other than helping initialize the pose estimation for their follow-up keyframe). It might be worth trying to fuse their depth data with the closest keyframe to possibly obtain denser and less noisy depth in keyframes (while however also introducing possible occlusion issues with this reprojection, if something is projected into a keyframe which is actually not visible from its viewpoint).