announce zero progress directly after the very first byte has been read
For some scanners, announcing zero progress directly after the initialization is suboptimal as they do not send the image data directly during scanning. Especially, with the network escl backend, the image is only send after the actual scanning it seems. By delaying the zero progress signal until actually reading the very first byte, Skanlite and Skanpage can react appropriately for the time period where the scan has been started, but no image data has been received.
Would like to backport to stable.