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,

1. The layer texture does not need an alpha channel, which results in reduced texture size. (done)
2. We can do the blending ourselves, then disable the blending for the shader effect. (todo)