D23021: [Image Wallpaper] Use layer.enabled during transition to avoid background glimpsing through

[Kai Uwe Broulik]

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REVISION SUMMARY
  Opacity affects each item individually, so during the transition, you have an `Image` with a certain opacity and a `Rectangle` behind it. Since the `Image` has its `opacity` transition to zero, you can see through it to the background `Rectangle` behind it.
  Using `layer` during the transition we can combine the entire item into a single surface and animate its `opacity` as a whole.
  
  BUG: 410313
  FIXED-IN: 5.17.0

TEST PLAN
  - Wallpaper transition doesn't flash anymore. Previously was most noticeable when just changing the size mode (e.g. from stretched to centered)
  
  Not really sure about the performance implications of using `layer` on such a a large surface?

REPOSITORY
  R120 Plasma Workspace

REVISION DETAIL
  https://phabricator.kde.org/D23021

AFFECTED FILES
  wallpapers/image/imagepackage/contents/ui/main.qml

To: broulik, #plasma
Cc: plasma-devel, LeGast00n, jraleigh, fbampaloukas, GB_2, ragreen, Pitel, ZrenBot, himcesjf, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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