display picture first and then generate thumbnails

Submitted by karamoro

Assigned to Igor @f2404

Link to original bug (#16188)

Description

when i open an image it takes a long time if in folder there are hundreds of pictures. this happens because the thumbnail generation is running and only after all pictures in folder have been processed the picture of interest is displayed.

please make the main picture display first and the thumbnail generation second. this way you will get the view of desired picture almost instantly.

Version: 0.8.3