Keeps it from loading the Svg and all the Plasma stuff (Theme etc) that comes with it on startup.

Found this while investigating a crash in KWindowSystem (used by Plasma Theme).
Reduces RAM consumption after kwin start by ~1 MiB, didn't measure time
Edge glow for screen edge and panel still works

I chose to un-const cornerGlowSize rather than doing a const_cast. I noticed that the glow size differs between OpenGL and XRender but this is without this patch also.