qt: fix effect behind the initial scan progress bar sometimes broken on wayland with fractional scale

TextureProviderItem does not emit dprChanged() signal because Qt apparently does not forward that to items that do not have ItemHasContents flag set.

This fixes the frosted glass effect looking broken at start on Wayland with fractional scale behind the scan progress bar (when it is immediately shown upon start) if the progress bar is shown before the fractional scale is retrieved (where the ceiled value is used in that case, but never updated).