picture: fix attaching props to delayed output pictures

If a Metadata OBU appeared right before a Frame Header OBU with show_existing_picture = 1, it was not being attached to it but to the next assembled picture, which was in the following TU.

An easy way to test this is by using the showinfo ffmpeg video filter to show attached side data in frames with the sample in https://github.com/AOMediaCodec/av1-hdr10plus/wiki, where you'll see some frames are missing the HDR10+ metadata before this change.