slowly load a big net img

[385841539]

when i load a big net img, it is very slowly ， so could given a param 'thumbnail' img url for user , the thumbnail url is generate by the original img by the user .
load the thumbnail first ，beacuse it is small ，so load fast . when original img loaded , replace the thumbnail img ， and give a scaleStateController by the thumbnail img which is scal by operator.

[leonacky]

try native photo view https://pub.dev/packages/flutter_photo_view