st: Rename and fix blur sampling calculation

st: Rename and fix blur sampling calculation

What the blur shader does is going n_steps in each direction (e.g. in case
we're in the horizontal pass that means left and right direction), sampling
the adjacent texels of the texel we're currently blurring. That means
n_steps actually is the amount of texels we're sampling in one direction,
not in both directions.

Make n_steps match what the blur shader does, and rename it to sampling_radius
to match what it really means. Do that for both st-theme-node-drawing.c and
st-private.c

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1542>