Updated the blur method to use the more efficient dual kawase blur algorithm.

Summary:
Updated the old and outdated blur method to use the much more efficient dual kawase blur method.
Now with this we can do virtually infinite blur with very very little performance cost.
The dual kawase blur method is basically downscaling and upscaling an image, but combined with the kawase blur shader.
Comparison: https://i.imgur.com/mh6Cw61.png
Left is old, right is new.
Comparison was done with the strongest blur setting in a VM running on an Intel i7-4790 and a GTX980
We can see here that the performance is even better with this new method.

Reviewers: Plasma, KWin, graesslin, fredrik

Reviewed By: fredrik

Subscribers: hein, dos, luebking, broulik, romangg, zzag, anthonyfieroni, mart, davidedmundson, fredrik, ngraham, plasma-devel, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D9848