Make mask smaller on corners to prevent broken pixels

This takes the simplest approach possible to fix the broken pixels in the dark theme at the corners: making the mask have a smaller corner radius compared to the actual element; by making it a bit smaller, it will not exceed the area of the applet and draw those broken pixels.

Since the plasma theme is transparent, it's still not the perfect solution; however, it looks much better than before and - if you do not zoom in - there's no visible bug on the corners, at least on my eyes.