thunar doesn't generate preview of existing PNG image in file conflict resolution dialog

[Karl-Philipp Richter]

steps to reproduce:

  1. somehow create the situation that a target directory contains an image without a generated preview
  2. move a copy of that image from another directory into the mentioned target directory

current outcome:

  * the dialog asking whether the move should be cancelled or the target overwritten displays the icon for missing preview for the target image which makes it hard to decide whether the target ought to be overwritten

expected outcome:

  * the preview of the target image should be generated

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: thunar 1.6.11-1
ProcVersionSignature: Ubuntu 4.10.0-26.30-generic 4.10.17
Uname: Linux 4.10.0-26-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.4-0ubuntu4.1
Architecture: amd64
CurrentDesktop: XFCE
Date: Sun Jul 9 20:15:25 2017
InstallationDate: Installed on 2015-12-12 (575 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
SourcePackage: thunar
UpgradeStatus: Upgraded to zesty on 2017-05-01 (69 days ago)