css shadows: Cache blured masks for the corners of outset shadows
css shadows: Cache blured masks for the corners of outset shadows

A lot of time was spend rendering the shadows on windows with CSD, in
particular the corner pieces, since they are the largest parts. This
patch catches this particular case and caches the pre-rendered blur
masks.

This makes the shadow code go from 25% to 8% of the time when resizing
gtk3-demo.

jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,jigpu/gtk,jigpu/gtk,alexlarsson/gtk,jadahl/gtk,jadahl/gtk,alexlarsson/gtk,jadahl/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,davidgumberg/gtk,jigpu/gtk,grubersjoe/adwaita,jadahl/gtk,davidgumberg/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,alexlarsson/gtk,davidgumberg/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,davidgumberg/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk