qt: fix color banding issues with dithering

Currently we are suffering from severe color banding when the dark theme is used. When premultiplied colors are used, we can't really use the whole 8-bit colors, as the dark color's parameters can end up getting so small that 8 bit precision would not be enough to represent them properly anymore.

For example, if color RGB(0.05, 0.05, 0.05) has alpha value 0.1, it becomes RGBA(0.005, 0.005, 0.005, 0.1) when premultiplied. I doubt 0.005 can be represented properly with 8 bits.

Close #27897 (closed).

Fading edge before:

Fading edge after:

Shadow before:

Shadow after:

Request review @chub.