desktoptheme: Use more moderate contrast parameters

With the current contrast parameters, the brightness of background will be increased. The biggest issue with it is that it results in flashing with the dark theme. In a dark environment, it is much more noticeable. This also excerbatted the infamous "korner" bug, but to be fair, we still need a more technically correct solution for that.

In 6.5, we made an attempt to unify the background contrast and the blur effect. There were a couple of reasons to do it, but the main ones were: preparation step for the background effect protocol, improve performance, and in general, make the blur effect more consistent. Unfortunately, the users didn't like the visuals, mildly speaking. After a few back and forths, we reverted things almost back to the same state that we had in 6.4, but we still made a few interesting (emperical) observations. The main one is that users liked a slightly saturated look of the blur effect.

After making the contrast effect opt in again in 6.5, this introduced the flashing issue back. For 6.6, we gave a shot to a different modulation curve but due to its very steep shape, this regressed the logout greeter (the background kicks in too abruptly now), so we are back to square one.

The reason plasma uses quite somewhat extreme background contrast parameters is to be smart about the background behind it. So if you have a pitch black wallpaper and a light panel, the panel background still looks bright, and so on. While, yes, it seems like a reasonable concern, it's also a bit a corner case and we did ship plasma with normal background brightness without any major backlash from users (in fact, folks expressed their preference for those new looks).

This change tweaks the background contrast parameters to address the flashing issue. With this, the background brightness won't be increased. However, the background saturation will be increased a tiny bit to make background appear more vibrant. The proposed default is not too high. By going with more moderate parameters, we also open up the road for a more softer transition, for example the blur effect can increase the saturation for all windows, not just plasma ones. The differences will be visible but in small steps rather than huge steps (which played a big role in the fallout of the 6.5.0 release).

The proposed defaults should also be more forgiving towards us with regards to fractional scaling. There will be still issues but you'll need to look much closer to notice them.

Before	After

Some visual differences are expected, but even with that, Plasma still looks really really good.