qt: attempt to fix color banding - request 30-bit color depth

We use colors (notably in shadows) that is not possible to be represented by an 8-bit number.

Qt uses pre-multiplied alpha representation, this reduces the color depth to below 24-bit.

Consider the color RGBA(0.05, 0.05, 0.05, 0.1), this becomes RGBA(0.005, 0.005, 0.005, 0.1) when pre-multiplied. 0.005 is out of the range of values that can be represented in 8 bits.

If 30-bit depth request fails, it should use the default 24-bit depth.

Closes #27897 (closed).