progress: fix missing last progress from loading layers

[tonistiigi]

Because of the bugs in the logic of how layer progress is throttled the last event may never be written, leaving behind incomplete output like

 => importing to docker                                                                                   0.9s
 => => loading layer f0074c45c2c0 190B / 190B                                                             0.9s
 => => loading layer 0e8819756174 393.22kB / 38.08MB                                                      0.8s
 => => loading layer 63ec8d9b4ac7 144B / 144B                                                             0.0s

Fix by always updating the current values, even if an event is initially throttled and not shown, plus make sure it always gets updated in the map.