st: Use blur node to render shadow pipeline

st: Use blur node to render shadow pipeline

It is generally faster than downloading the texture, blurring
with the CPU, and uploading the blurred texture. Notice that
'n_values' is calculated differently from the blur_pixels()
function; that's because it needs to match Clutter's blur
shader interpretation of sigma [1].

[1] https://gitlab.gnome.org/GNOME/mutter/-/blob/master/clutter/clutter/clutter-blur.c#L89

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1539>