qt: fix sub-pixel font rendering when applicable in FadingEdgeListView
Fixes #27472 (closed) partially.
Sub-pixel font rendering is still unavailable, when the QQuickWindow
is transparent, and there is stuff showing behind. It could be the embedded video, or compositor inter-window blur feature.
This can also be a performance improvement, since if the (fully opaque) background color is determined beforehand,
- The layer texture does not need an alpha channel, which results in reduced texture size. (done)
- We can do the blending ourselves, then disable the blending for the shader effect. (todo)