blur-effect: Implement incremental calculation of gauss coefficient

blur-effect: Implement incremental calculation of gauss coefficient

Use the shader for linear sampling and incremental calculation of the
gaussian kernel values as it was implemented by Patrick Walton in
webrender.

The sigma value for the blur (the standard deviation) is calculated by
taking the blur radius and dividing it by 3, this value is used by most
implementations of gaussian blurs since it covers a high percentage of
the gaussian shape.

The linear sampling optimization is implemented by skipping every second
texel (i += 2) in the for-loop that's sampling adjacent texels.

https://github.com/servo/webrender/blob/master/webrender/res/cs_blur.glsl
https://github.com/servo/webrender/commit/38ec7db6f165ff7aaad12d91eca7d7a5f51557b5

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/991