st/theme-node-drawing: Don't skip square top left inset shadow corners

st/theme-node-drawing: Don't skip square top left inset shadow corners

When the inner radius of the top left corner is 0, the corresponding arc
does not get drawn, resulting in a path going from the bottom left
corner to the top right corner when the path is closed.

Fix this by always including a line_to from the bottom left to the top
left corner.

Fixes: dde13b30c ("st/theme-node-drawing: Trace the correct path for inset box shadows")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7996