D10223: Improve preview thumbnail quality

[Eike Hein]

hein created this revision.
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REVISION SUMMARY
  This patch does two things:
  
  - Instead of giving the KAbstractViewAdapter the actual icon size, we overprovision by giving it the size times two. This is because while we're deriving the grid view cell size from the icon size setting, it's not the actual size the thumbnails will be displayed at. Our IconItem is sized to almost fill the cell, and for image sources with non-square aspect ratios IconItem doesn't scale to the next icon size.
  
  - We set IconItem to do smooth scaling.
  
  While this results in lovely visual fidelity, I the performance
  impact is a concern. We're requesting twice as large thumbnails
  now, and we're doing more scaling work. However, thumbnail
  generation is async and doesn't slow down listing. There's also
  the possibility IconItem::smooth could have general performance
  impact unless things are smart enough not to scale when not
  necessary, which is hopefully the case.
  
  BUG:376848

REPOSITORY
  R119 Plasma Desktop

BRANCH
  Plasma/5.12

REVISION DETAIL
  https://phabricator.kde.org/D10223

AFFECTED FILES
  containments/desktop/package/contents/ui/FolderItemDelegate.qml
  containments/desktop/package/contents/ui/FolderView.qml
  containments/desktop/plugins/folder/foldermodel.cpp

To: hein, #plasma
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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