qt: fix non-smooth alpha animation with temporal dithering

This is akin to !6678 (merged) where I intended to fix color banding.

With the same cause (not having 8-bit precision with pre-multiplied alpha), we have the alpha animations not appear smoothly. While I'm not sure if this is the same reason, I have found a Qt bug here: https://bugreports.qt.io/browse/QTBUG-85566.

I have observed this problem when I added many items to the playlist and quickly swipe the cursor over them in dark theme. For me, this is already an issue with the linear easing color animation that has 200ms duration, but some may notice it with longer durations (such as 1000ms or 2000ms).

Request review @chub.