Inpainting preserves the INITIAL latent outside the mask. The node was
handing it the empty latent, so everything outside the mask came back grey
instead of the source video -- the encode that a normal inpaint graph does
before Set Latent Noise Mask was simply missing.
The guide chunk is now VAE-encoded and becomes the sampling latent whenever
a mask is in play. On AV latents the audio stream from the connected latent
is kept and only the video stream is replaced.