This is a moderately fast two-pass gaussian blur implementation.
It downscales the framebuffer dynamically before applying the
gaussian shader, which cuts down rendering time quite considerably.
The blur shader takes 2 uniforms as input: the blur radius; and
whether to blur vertically or horizontally.
The blur radius is treated as an integer in C land to simplify
calculations. The vertical parameter is treated as an integer by
the shader simply due to Cogl not having proper boolean support
in snippets.
At last, brightness is also added to avoid needing to use an extra
effect to achieve that. Brightness is applied in a different pipeline
than blur, so we can control it more tightly.
ShellBlurEffect also implements a "background" mode, where the contents
beneath the actor are blurred, but not the actor itself. This mode is
performance-heavy.
Related: https://gitlab.gnome.org/GNOME/gnome-shell/issues/1848https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/864
