(Yakuake) Fix black squares at rounded edges

[Lorsoen]

This PR aims to fix the black artifacts near the rounded edges of both light and dark versions of the theme :

This uses a pretty gross hack, consisting of setting the shadow's ending opacity to 1 percent instead of zero.

There's no noticable changes to my eye (except for the now gone black edges) but feedback on other setups is welcome :

This bug happened to me on Plasma 6.1.5 with Intel graphics. I am not aware if it could be fixed by porting the theme to Plasma 6, or by using different drivers.

Assuming it's gonna take a while only to receive feedback on this, porting is not gonna happen in a while or ever, thus the aformentionned hack :sweat_smile: