if compositing is available, show a subtle glow where the hidden panel is when the pointer gets within 30px of the trigger area.

the idea is to give a hint as to where the hidden panel is as we only trigger on its foot print, not the whole screen edge. and... well... it's a cool idea. =P

the mechanics actually work without compositing except the painting of the glow, which looks like ass because it really requires proper blending to look nice.
so no fancy for the compositeless.

re-using the button glow artwork for now, should probably be using something designed specifically for this.